Don't Make a Silly Mistake

Steamboat Willie is now in public domain.

Mickey Mouse?

That's a Trademark.

If you think that the Disney lawyers who specialize in trademark have not been preparing for this, you may be making an expensive mistake.

I have a graphic design degree but I'm not a copyright artist, so the following is not legal advice, it's just a set of observations:

Steamboat Willie vs Mickey Mouse

On the left is Steamboat Willie, on the right is Mickey.

Biggest design difference: Mickey has gloves, and Willie doesn't. Don't draw Willie with gloves, if you do, you're drawing Mickey. You can't see it in this particular screencap, but Willie does have fingers, so you can draw him with full hands.
Willies eye's are black solid ovals, with no sclera and no eye-shine. Mickey has white sclera, smaller distinct pupils, and usually has eye-shine.
Willie is in black and white. You could theoretically make Willie whatever colors you want but for safety, you should obviously avoid Mickey's iconic red shorts with white buttons and yellow shoes.
Mickey's face is usually colored peach these days but it can also be seen in white.
Mickey's shoes tend to be drawn bigger and rounder than Willie's.
Willie doesn't talk, so he doesn't have the classic Mickey voice, so if you're doing like, a voice-acted video... just don't try to imitate Mickey. Give him a different voice.