Using a autobuyer is really easy.
Say you want to search Ramires from chelsea. Search on the web app or console his lowest buy now. if his lowest buy now is 5k try to buy him for 4.3k in any formation and then sell him for 4.9k. That's really it you can do the same with any player in any formation.
If the cheapest player on the market is 6k i will probably try to buy it for 5.3k

. And try to search the players in popular formations like 3-5-2 , 4-1-2-1-2, 4-3-3, 4-4-2
I hope this helped you