This is a very simple tactica explaining the use of Lootas in an ork army. Lootas are essentially an ork carrying an autocannon. This gun instead of shooting 2 shots it shoots d3 shots. These guys are 15 points a model and are pretty much the equiv to the point cost of just the gun for other armies. The main drawback is the lower BS of an ork making the shooting much worse. GW's solution to our poor shooting was making squads of up to size 15 of these guys...so up to 45 shots at s7 ap4 per squad!!! Even with ork shooting you will kill stuff with that.
Lootas take up and elite slot and unlike termis, ork nobz will be taken as troops in conjunction with a warboss. This means ork elite slots are wide open for spaming lootas. Some people run squads of 5 or so and multiple of them. I like that strategy so I can fire at more targets but I also like running large squads to make them fearless and avoid taking morale checks. Yes I fire at fewer targets but my squads last much longer.
Now that you have chosen to take lootas another questions is "will you also take grots"? I take grots as a really cheap meat shield to block incoming charges by fast squads, deep strikers, out flankers, and infiltrators. A perfect example is a 20 man genestealer squad coming onto the board and trying to take out the lootas. My grots blocked them and the lootas took the squad out!!!
The next part is placement. If at all possible place them were they can cover important fire lanes and at the same time sit in cover. A 4+ cover save will really make these guys hard to kill. Once you are in terrain try not to move and just blast away with your big guns. It is that simple, lootas pump out fire power and are the best long range orks in the codex. Lootas are a huge distraction for any mech army as they just rip open any low armor tank and sit in cover. One last note is lootas should be used to open up tanks that you expect your squads to be charging. Get the squishy goodness out of the tough shell and then eat it!!!!