Step 1: Draw the body
Using Illustrator or After Effects, draw the character. I prefer Illustrator because it's pen tool has better control. Using a reference image, I recreated Finn's body. If you use Illustrator, make sure to keep each body part on a separate layer so you can animate them separately in After Effects. For best practices, name each layer accordingly so you know what you're selecting.
Step 2: Import into After Effects
Create a new composition in After Effects. Import the AI file into After Effects by going to File>Import>File then select the AI file, set "Import As" to Composition - Retain Layer Sizes and click "Open" so that you import each layer at their original size, making it easier to select and animate each body part.
Step 3: Draw limbs in After Effects
Draw the 2 arms and 2 legs using the pen tool, adding a curve so you can edit the arc of the path for the walk cycle. For the back arm and leg, make the shade slightly darker to distinguish the difference between front and back. To create the sleeves, shorts, socks and shoes, simply duplicate the arm or leg paths and change their colors. Then, in each layer, add a Trim Path and change the values for start / end to what looks correct to you.
Next, create the character body part hierarchy by parenting each layer so that when the torso moves, everything moves with it automatically.
Step 4: Rigging
Since this is simple animation without bones or plugins, all you need to do is make sure the anchorpoint of each body part is where it needs to rotate (ex: arm anchorpoint is at the shoulder). When a layer is selected, press Y on your keyboard for the anchorpoint to show, then click and drag it to where you need it to be.
Step 5: Animate