hmm it depends on the CG like Toru and his cat costume CG for example you need to buy the cat costume and in the CG he's a submissive bottom so you make him a bottom and make sure u click the heart button and then you should unlock the CG, I'm sorry i don't remember enough to make more of a guide, but honestly just look at the CG you want and then do what the CG shows and it should work.
Where do I find out what CG's are in the game though? I haven't seen it in the guide and can't find anything on the internet. With Shuu, for example, I finished his route but am still missing lots of CG's and don't know how what CG I am missing. Plus it was confusing because I thought every single costume creates a roleplaying scene, but it seems like there's only specific costumes for specific characters - Maid for Toru, Doctor for Shuu, Dog for Shiba, and Gakuren for Tocchan, but I'm wondering if there's more costume roleplays than just one in each route. It would be nice if there was a guide for how to get each CG though. Don't you know when the game developers might make one? Or is it already out somewhere? Thank you.
I know when the game was still coming out the devs said they were working on a guide that they could sell but now apparently it doesn't exist? I'm sorry to say I don't have a guide for you but I think if you go back about 10 months in the main pages comments, it was very active back then because the game was just released and I think you can find a lot of help there. There should be people asking how to unlock some CG's and also answer's and possibly screenshots of CG's you might not have.
Around here there should be a lot of advise on unlocking CG's
https://meyaoi.itch.io/seiyuudanshi/comments?after=460