First you need to copy paste example mod to user mods folder (its near your saved ships folder).
Second you need to take a look at mod.rules everything there should be commented enough for you to understand.
Third check other files of mod, especially part definitions they contain more comments on how you should make what you want.
Fourth if still in doubt how to make something, check if it has been done in vanilla game in some shape or form, for that check Data folder of game.
Fifth go nuts.

Now that outlining steps is done, for simplicity, this guide will use creation of armor part as starting point.

As basic armor template is already present, you can start modifying it. First, change the ♥♥♥♥♥♥♥ id of part, then you should change strings attached, but thats not necessary.
Next are sprites.
Here is list of sprites to make if you want your part to not look like made by caveman (some are not needed for armor, but you still can use them to make it look better):
Icon
Blueprint
Walls
Floor
Roof (this gonna need normal maps tho, if in doubt use blank ones)
Blueprint sprite should be tinted blue, but if you get colors bit wrong, its nothing fatal. As an option you can waste time and add damaged sprites, but its better to use same time to make quality doodads with animations for part, cuz players rarely look at damaged parts. After you add/modify text in strings en.txt, you done with armor. Speaking of doodads, here is list of additions to make your part look better:
Doodads
Lights
Windows
Roof doodads (yes, its not windows)
Animations

This should be enough to dip your toes into mod development at a beginner level.
Have fun.