Best way to get A Grade transfers

It's been a long time since I played manager mode, usually sticking to Ultimate Team for the most part, but had been looking to get back into it, certainly with rewards behind objectives for all game modes.

I was wondering if there were any tips on how to get an A Grade in transfers or if there was a guide on the best players to use for this? No matter what I try, it's always coming up at B or lower.

Any advice would be great!