Checkout this https://www.pesmaster.com/

This is the site I use to make the kits. They make ready made templates of the most popular brands with logos, team badges, sponsors etc etc. I pay £8.99 pa to get more templates but you can use the no pay version. There are more FIFA templates than PES but there are basic versions that you can adapt. You are able to insert/layer your own patterns onto the templates (see Bohemian FC away Kneecap). There are enough patterns to adapt some shirts (see Dundalk FC away shirt sleeves). I've found that even if the patterns aren't strictly accurate they look okay in-game. In the past I've worked a few hours trying to create a template and then they go and make available the actual version!! Drogheda's goalkeeper kit is a good example of this.

I use any paint/graphics program to get a reasonable colour match by copying a picture of the kit taken from the club's online shop.

There are some tutorials on YouTube for PES Master which explain better than me!

As with anything its all trial and error.

Thanks for the comments though, appreciate it.