Thanks

Modifying another publicly available skin for your own use is perfectly fine, but posting the result (especially here) without the original artist's permission is not.

You could, of course, upload skins containing only your own work (like a new shirt) that are intended to be layered on top of existing skins.

From what I know if it isn't similar to the og one I know its ok, but don't copy it too much

No, unless the skin was intended to be used as a base or if you have permission from it's creator to use it, so something like this: https://www.planetminecraft.com/skin/rainbow-flower-crown-base/ would be fine to use, but something like this: https://www.planetminecraft.com/skin/cat-maid-base/ which mentions not to post edits of it would not.

If you are uncertain then I would suggest asking the person who made the original skin, if they say it can be used then it's fine to use.