I've seen a lot of videos, articles, and blogs suggesting the best ways to build Gunpla. I really can't give singular credit to any source, but I have drawn off of many sources in my own model adventure. The most important thing you should always remember when working on your kits is that these are works of art, they're plastic sculptures, and as such they deserve your very best effort. Under no circumstances should you ever rush a model. Always try your hardest on each and every piece to clean them up to your satisfaction, do not skimp on this step.
I view the initial snap fit and construction as the foundation upon which the rest of the model has to be built. Any flaws here will show in your final product and can end up making you feel like you just wasted weeks of time, materials, and money. The inverse is also true - putting the right amount of effort in shows, people notice it, and the feeling of satisfaction is more than enough to make you feel like your time and money was well spent. So try your hardest, take breaks, and do not try to finish the kit you're working on solely to start the next one. It's tempting, but do your kits and your skills some justice; you only build them once but you will have them for the rest of your life.
The first step to building a good model is getting the right tools.
This is really the most basic tool. There are times when you simply must lay a piece flat and trim with a knife. This can leave really unsightly gouging in your work surface which is not ideal and is especially upsetting when it's a nice table. Invest in an affordable cutting mat that's a decent size. It's also a very good surface to keep all of your pieces on as you work and most double as a ruler for when master grade kits occasionally ask you to trim pieces to a certain length. I also collect all of my nub trimmings and shavings on my mat by sweeping them together with a flat file. When I'm done, I dump them in the trash and my work space is clean as a whistle. Don't deny the cutting mat, use it to your advantage.
My Tool: X-ACTO self healing cutting mat (12" x 18")
While the mat is the most basic, cutters are the most crucial. A decent pair of side cutters can make building a model many folds easier. What you're specifically looking for is a 'side cutter for plastic.' It looks like a delicate wire cutter but what makes it special is that it's completely flat on one side of the cutting head. This allows you to be very precise in your cutting because you know exactly where the blade is going down. You push your cutters very snug up to the piece during clean up and you can sometimes cut so close you don't even need to go to your craft knife. It speeds up the process, it's gentle on plastic, and in the end will provide you with a much higher quality model. I use a Tamiya side cutter for small, delicate work and a Xuron side cutter for larger work, such as cutting pieces off the plastic tree initially.
My Tool: Tamiya side cutter (from the Tamiya Basic Set) & Xuron 410 Micro Shear Flush Cutter
A craft knife is your first defense against unsightly remnants of the plastic sprue that the modeling community refers to as nubs or flash. Modelers allow for a small amount of flash to remain on the piece because cutting too close with side cutters can occasionally damage the surface and tends to stress the plastic too hard, leaving ugly white marks that you can't remove without paint. I go in with my thumb reinforcing the back of my blade and gently push through several times until there is very little remaining flash, in the same kind of method as shaving the bark off a piece of wood.
Take care to avoid the very common (and very nasty) mistake of surface gouging in which you cut too deep when trying to remove a nub. It's always better to leave more than you need to later sand it down than getting too aggressive with the knife early on and ruining a piece. Fixing this requires putty and paint, its best to just avoid it. I don't use X-ACTO blades for my knife, I use a retractable, snap off hobby knife. I like it because it allows you to hold very close to the blade, it allows you to choose blade length, and you can easily resharpen the blade by just snapping the top segment off.
My Tool: Tamiya retractable craft knife (from the Tamiya Basic Tool Set) or a 'Light Duty Snap-Off Knife' works as well
Metal file set
When I first got back into Gunpla a few months ago, I only had one small metal file with very little tooth. I thought it worked great, it was gentle and slow, but I really was missing out. I invested in a $9 set of three flat files from Tamiya and it totally changed the speed at which I build. They're fairly thick tools, dark metal that feel a bit heavy, but it only takes a handful of swipes over a nub until it's ground down just right.
A flat file is incredibly efficient at keeping the angle of the surface you're sanding - what I mean by this is that other methods of sanding that have you come from numerous angles can shave down a side of the model that is supposed to be flat but instead has an odd, rounded corner or uneven surface to it. With a flat file, you just lay it on the surface you want to work and then gently pull or push across the surface in a singular direction, keeping the initial angle. Going back and forth can contribute to the rounded edges and loss of flatness because you rock your wrist (changing the angle) and shave it off unevenly. Keep a locked wrist, only go in one direction, don't use much pressure. Files leave a very rough surface which is fine because it will be cleaned up later by sanding pads.
My Tool: Tamiya Basic File Set (includes three various shapes of files) and Tamiya mini file (from the Tamiya Basic Set)
Often overlooked, sanding pads are what separate your model from being good and being great. You want them for the obvious reason of cleaning up the mess your flat file leaves behind as far as smoothness is concerned. It's best to have a few grits of sanding pads - one that is less rough than your flat file but still fairly rough and one or a few of varying smoothness. Pads are better than paper because you can get a grip on them, they conform to the shape better, and they last a lot longer than paper. You can always glue sandpaper to sponges but the pre-made stuff is really nice quality and will last through many models.
With your finest grade sanding pad (1500 - 2000 grit), you can buff the plastic in small circles which has a couple beneficial effects - it reduces highlights on the surface of the model which tends to look less 'plastic' and it also aids in removing or reducing the unsightly effects of Bandai's injection molds which can leave an odd, uneven, whirled pattern on the surface.
My Tool: Alpha Abrasives sanding sponges for models, but you can look for 3M sanding pads from Superfine to Microfine, or look for Revell sanding pads (smaller ones in picture)
And here you thought Gunpla required no glue! This is actually a bit deceptive because the glue doubles as a seam filler, often times taking the place of putty altogether. Kits often come with weapons which don't even attempt to hide seam lines - with your trusty glue bottle, brush a fairly thick coat of glue on the inside of both pieces, slap them together, let them dry for 24 hours, then sand down. Once you sand all the glue goop off, you'll be shocked to see that the piece has essentially welded together and left no trace of a seam.
My Tool: Mr. Cement Deluxe Economy Bottle (40ml)
Perhaps the least crucial of all, tweezers can make your life much easier when working on small stuff or applying decals. Sanding tiny pieces, such as the mono eye ring on a 1/144 Zaku F2, can be made a lot easier with a fancy pair of reverse tweezers. They always hold tight, so you only push them together to fit a piece in and they do the rest of the work. Good idea to have some, but not absolutely necessary. I'd suggest eventually picking them up though, you'll find a use and they're not that expensive.
My Tool: Generic reverse tweezer (search online or ask at a store) and Tamiya regular tweezers (from the Tamiya Basic Tool Set)
Tape is truly most useful as a system for identifying plastic sprues. It can be quite bothersome to have to search through a bunch of plastic trees to find the tiny letter in whichever unpredictable corner it happens to be in. To solve this headache, I now put a small flag of tape on the side of each sprue and label it with a permanent marker, A, B, C, etc. You'll be shocked how this little step can speed up your process.
My Tool: Generic roll of white masking tape
As mentioned above, they're useful for marking off the tape so you can easily identify sprues, but colored Sharpies also make incredible tools for coloring transparent plastic like a Zaku's mono eye. Test them out on the clear part of the tree and use whichever color suits your fancy.
My Tool: Sharpie color set - make sure to find something with a magenta or pink color as most Zakus need it! Green is also a very common color.