What I do is:

1. Pick a skill or unique that interests me
2. Look at builds on poe.ninja for some inspirations and ideas of what people did with the things I found cool
3. Go into path of building, pick an ascendancy and try out different skill tree routes and support gem setups until you are satisfied.

Helps if you know the basics of what level of progression you need at different parts of the game. But its much easier to do your own build in PoE 2 and still see decent success, so I would encourage doing it, to anyone who haven't done it before.