It's easy as can be, and really, the hardest part was waiting for layers of paint to dry.
A toy or several
Acrylic paint in whatever colors you'd like (put down a base of gray acrylic paint if the toy you're painting isn't already gray)
Cup and water
Glitter or pearlescent powder
Fine grit sandpaper
Wipe down your toy with a paper towel. Optional: if you want to give the surface a little grip, go over it once with a fine grit sandpaper.
If your toy is not already gray - both of my miniature AT-ATs were - paint it with approximately two coats of acrylic paint so there's a nice uniform base. Let the paint dry completely between coats.
I'm sure spray paint would also work fine and probably look cleaner, but you need to use spray paint outside and it's cold where I live so that's not happening.
Once you have a gray surface, painted on or otherwise, add pretty colors. I realized a foam brush is not ideal for this project since it sucks up the paint, but any other paint brushes should be fine. I recommend using both medium and small sizes. Add the paint in thin layers, waiting for each layer to dry completely before moving on. Don't layer on too much paint or the paint will cover up the details (you can see where I did that with the pink AT-AT). Unless you're more talented than me, you're going to get paint on your fingertips.
When you're happy with the opaqueness of the paint, clean your brushes and admire your new brightly colored AT-AT army. If you want to apply glitter or pearlescent pigments, shake it on while the paint is still wet (or you can buy glittery acrylic paint).
If you're feeling ambitious, you can go back and add shadows and/or highlights once the toy is dry. I tried to add some texture to my gold R2-D2 with a little bit of black acrylic paint mixed with a lot of water (I brushed a lot on, let it sink into the nooks, and wiped most of it off with a paper towel).