Once you have all four parts how do you assemble your cannon on runescape is there a certan order or something?

Answer

Jagex updated and now you can just put the cannon base down and it assembules it'self every 400 shots your cannon will decay but you will get a message saying "your cannon has almost decayed!" and you pick it up and it restarts the decay. 1 steel bar=4 cannonballs but I recommend just buying the cannonballs because making them takes a while