I never tried tanking before. But when I tried to lose a game e.g. in some opponent initiated friendly matches, I did what santansboss mentioned above - move players to both sides of the field and arrange a non-goalkeeper as goalkeeper.

I am also considering tanking now because I want to take some time off. I think I will sell all old players and buy some young players to develop. I don't want to do too much in order to lose league games because this may not be fair to some players.