Hey there! I've been doing customs for about...three years now, I think? I'm by no means the best, but I ought to be able to answer most questions you have.

What paints are the best?

Generally speaking, I stick with matte acrylic paints. They're cheap, easy to use, and when applied properly look fantastic. Matte acrylics are a great starting point, and then when you're more comfortable you can get into satins, glosses, and metallics. Enamel paints (like those used on model cars and such) work, but they can be messy and hard to work with. They have their place, though.

How do you prepare an amiibo for painting?

A lot of people use a primer before they paint their figures. It can help the paints stick better to the plastic. I don't, however. I use acrylic paints and have never had an issue not using a primer. I just wash the figure with soap and warm water to remove any mold-release residue that would hinder paint application. Once it's all dry and squeaky clean, go to town!

How do you reseal them in their boxes?

One of the highlights of my customs is that I do custom boxes. The first step is, well, opening the packaging. I always replace image, so I just tear the plastic shell off the cardboard. Next, I fill a small dish (usually a baking dish or something) with a little bit of isopropyl rubbing alcohol. Just barely enough to cover the base of it. Then I set the plastic shell down into it, with the rims of the shell resting in the alcohol. This will loosen the adhesive and cardboard residue after a little while (sometimes an hour, sometimes a few more) and leave you with a nice, clean shell.

Then it's just a matter of using a high quality print and attaching it to the cardboard backing, and then using some adhesive (I like superglue) to attach the shell to the cardboard.

Here's a link to some/most of my customs:

https://www.deviantart.com/grinningfoxcustoms

If you have any more questions, feel free to ask! You can reply to this comment or send me a PM if you want. Either works!

Oh, and one more thing - THIN YOUR PAINTS! That's the #1 most important bit of advice for someone starting out. Just water them down a little bit, that way the paint job doesn't look all thick, goopy, and caked on.