You control at least one additional nontoken creature.
Steps
Activate Phyrexian Altar twice by sacrificing another nontoken creature you control and Slimefoot and Squee, adding ({1} magic symbol) ({B} magic symbol) .
When the creatures die, Pawn of Ulamog triggers twice, creating two 0/1 Eldrazi Spawn creature tokens.
Activate Phyrexian Altar by sacrificing the two Eldrazi Spawns, adding ({R} magic symbol) ({G} magic symbol) .
Activate Slimefoot and Squee by paying ({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) and sacrificing a Saproling, returning it and another creature from your graveyard to the battlefield.
When Slimefoot and Squee enter the battlefield, it triggers, creating a 1/1 Saproling creature token.