Fusion monsters are purple. Fusion Monsters are placed in your extra deck. You activate Polymerization to Summon a fusion monster. Polymerizate 2 or more Fusion material monsters to fusion summon a monster
You have Dream Clown on the field, Crass Clown in youra hand and Polymerization in your hand.
You activate Polymerization. You send the clowns to the graveyard and polymerizate them to Bickuribox. Then, you summon Bickuribox from your extra deck and send Polymerization to the graveyard. There are more ways to Fusion summon, but this is the most common way. Also, you can use Metamorphosis, but it is not considered a fusion summon.
You can't special summon a fusion monster from your graveyard unless it's been fusion summoned to the field before it was sent to the graveyard.