How to add my own new part to KSP?

[frigid elm]

Hello! Could you help me, please? I didn't have any experience with modding of KSP, and now I want to understand a couple of things.

Can anyone explain, how to add my own new part based on existing one? I mean, I don't want to create a model, texture etc. I just want to duplicate an existing part, let's say a probe command module, and edit a couple of its parameters.

So I want to have two visually identical parts in the game but with different parameters. How can I do it? I understand, that I can edit parameters of parts using cfg files in Kerbal Space Program\GameData\Squad\Parts folder. What I do is I just copy a folder for an existing part, rename it, give it unique name inside a cfg file as well. So after this I expect to have two absolutely identical parts in part menu in the game, but I cannot see the second one. What do I do wrong?

[turbid shoal]

I would recommend to create a folder in gamedata, call it like "MyStuff" inside a folder "parts". Then you copy the part .cfg and put it in your folder. Rename tha cfg and open it. Then change name too.
Now you can change all valus you want

[frigid elm]

I tried it. The part appears in the game, but instead of having two parts, new edited part replaces the original one in the editor. However, when I launch a ship with my part, it appears to be the original one at the launchpad. What should I do to fix it?

[turbid shoal]

can you send me the config