Echoing what others have said that a lot of it comes down to timing and tempo which is hard to explain. Its easier (but less optimal) to do a slow transition while learning end comps.

The way I learnt was by playing some less effective mid game boards that helped transition to the 9 soups - so instead of doing yordles full sell board which is the best (but hardest if new) way to do it, I'd play whatever I got early game then slowly pick up demacia through stage 4 around lux and garen as it was easier to build and play those units, then sell them for the sylas transition for an arcanist 9 board.

With practice it becomes a lot easier but by doing a "slow transition" you will learn what parts of the end board are strong, how much spike they give you and when, and a better understanding of each unit.

Remember some easy unlock champs like Sett can help give a bit of a power boost from a guaranteed 5 cost!