Loki is a pretty contradictory kinda guy (He's cool though). He's generally depicted as a Jotun (giant), though was conceived when lightning (his father) struck an oak (his mother) and lives among the Aesir. He's Odin's brother and is pretty chill with Thor (they go back and forth between getting along and arguing).
When Norse beliefs were finally copied to paper by Christian commentators, Baldur was likened to Jesus while Loki was likened to Satan. Because of this, it's hard to find sympathetic depictions of him.
He is the patron of spies, trouble makers,plotters, double agents, and overall tricksters. He is associated with wolves, snakes, and sometimes spiders.
Since Loki is... a bit unreliable, invoking him can take time. With someone like him, leaving offerings of wine would probably be ideal. Making a formal alter could work as well, but he can easily become jealous or upset if forgotten. Give him his own space, and try to keep him away from images of Thor or Baldur if you can.
Also, definitely check out the poem "Lokasenna."