This isn't much of a step by step because the process is pretty simple and obvious but here goes.
These are my Adventure Time shoes. Total time it took me to make these was about 4 hours because I draw and paint slow :p

***UPDATE*** Added photos of my second pair of shoes on step 5. Much more comfortable and I think better overall****

Step 1: Buy the Shoes

Buy some plain white canvas shoes. I got mine from Walmart for around $5. They may be cheaper somewhere else like the dollar store. Remove the laces and hold on to them for later.

Step 2: Gather Visuals

Google Adventure Time or whatever else you want to put on your shoes and print the pictures you want to work from. These are the pictures I used.

Step 3: Draw

This is the most fun part! Draw your images on your shoes in pencil. Jake is really really really easy to draw. I may post another 'ible on how to draw him if anyone is interested, but it's pretty simple. When you have drawn your images on the shoes, go over the lines in permanent marker. I used one with an ultra fine tip. Going over the pendil lines in marker is helpful later when you paint, so when you paint over the lines you can still see them.

If you think you can't draw (you are wrong but) you can try using transfer paper but I think it would be more difficult than simply unleashing your inner artist.

Step 4: Paint

Now we paint! I used regular acrylic paints. Mix colors if you need to get the right shades. Paint Finn and Fiona, then the rest is easy because the rest of the shoes are the same color except the whites of Jake's eyes.

Note: for Jake's color I used Yellow Ochre mixed with Medium Yellow.

Now go back over the lines with permanent marker again. This makes the lines pop more since the paint dulls the black.

And finally, lace them up.

Now wear them proudly and show off your ultimate awesomeness!!!

Step 5: Newest Shoes

I took my 2 kids to Kangaroos to play while I sat there with my paints and made these. It took 5 hours total time. The kids were nice and exhausted :)

One shoe is Adventure Time and the other is Regular Show. The other side of the shoes have the various catch phrases.

I added the LSP and Peppermint Butler shoe buddies later. I followed the instructable by doodlecraft. If you aren't already following her, you should go do so now! I've got to make a High Five Ghost and a Skipps for the Regular Show Shoe.