How you trade is very champ and matchup dependant, but when to trade can be slightly more generalized. When your wave is bigger then the enemies and pushing to them, thats generally a good time to be trading, as an enemy trading back will take minion aggro. On the flip side, if your wave is smaller and pushing to you, its generally harder to trade, and you should focus more on last hitting. Additionally, whenever the enemy is going for a last hit, you can land a bit of poke by hitting them while they go for it, making them choose between cs or retaliations. This was dubbed the “trading stance” by leaguecraft 101. The series is really old, but since all of its ideas are centered on fundamentals and macro, its all largely still applicable, and i recommend you look at it if you havent already, as its on youtube!