honestly, for me, it's about the same as making any other character, just with the extra step of picking a role for them! ofc, given that they're cats and whatnot, there will be some other differences besides that probably, but still. is there a specific part that's giving you trouble?
Honestly just be creative, maybe look at some current warrior cat characters for inspiration and then make your own. I usually tried to base my early warrior cat character off of myself, it made things a little easier.
personally, i tend to come up with a character hook first just to make sure none of my characters blend in the background with the usual traits of "loyal, nice, caring, etc."
for example, my two most recent characters are the parents of a deaf kitten after the other kit in the litter passed in birth; the mother took it really hard and was really emotional over it, and the father was of course also heartbroken but still comforted her, and they both ended up completely adoring their deaf kit despite the fear that she might not be able to amount to anything in the clan ... the personality came really quickly after this original character hook, like characterizing the mother as very soft and curious personality-wise but also very prone to worry and sadness, whereas the father was characterized as being very ambiguous and mysterious to other cats, but to his mate who can read him very well, she loves his intelligence and the way he silently cares so deeply about those close to him
coming up with small things like that at first, like "this cat likes chasing leaves around in their free time when they think nobody's watching" or whatnot really helps me to make a character that's both fleshed-out and very lovable, the type of cat that really stands out in a clan of like, 20-30 other cats. i sometimes draw my cats, but more often than not i use pinterest (or this awesome person on tumblr) to find pictures of my cats that would match the clan that they're in, their personality, their lineage if their parents or grandparents are already established characters, etc.
this is just my process though, find whatever works for you! the most important thing about ocs is that you love them, after all (: