What you can do, go through your list of players, pick out the good ones (not necessary high quality = good), perhaps top 12 players (excluding the GK cos thats his only role), then see what formation you can come out with. Make sure that it can be flexible to adapt to some changes (i.e: ML/AML, DC/DL) so you can easily try out other formations if the current one isn't working well. Do adhere to the rules and make sure its not an Illegal formation.