I've been playing manual for a few years now. Before I switched to all manual, I was playing a mixture of semi and manual, which probably made the switch to full manual almost transparent for passing. Shooting and crossing came later but wasn't horrific.
I had passing on semi and through balls and lob passes on manual. This way you can play your regular game while you get used to aiming somewhat, while testing out full manual aims with the through balls and lob passes. It may be worth investing some time on semi-manual mix for the development and improvement before you go fully manual.
Playing against people using the same schema also helps. Obviously finding people playing full manual is extremely hard. Playing the computer is tough. There is no good difficulty level (it goes either too dumb or too impossible when you're on manual). You need to understand how to play around with sliders to make it a good game for your skills. There is a slider thread that has 1000s of pages to read to understand
which doesn't sound efficient obviously. It was easier to follow it as it progressed.