This all depends on how big....
Our team first started training on a field with a stream, and a 90 degree hill!
We got a few pallets together and built random base's over the field (not by any means good for tournament training though)
If you want to play tourny ball, and you have a fairly large garden then i would recommend getting some pallets, and some poles.
Simply bang the poles through the pallets and straight away you have a bunker. If youve got loads and loads of room, set these up at different heights (two pallets on top of each other) and use them like that.
If your limited for room, two bunkers will be still good facing each other - then just practice "snap shooting" between two people, Eg... both pop out and fire 3 balls at each other, then move quickly back in, the more you practice the better you get at accuracy, speed and playing tight. Or you could try the "posting" method, where one of you constantly aims at a bunker, and the other player has to snap shoot his way out, and then "post" on the other player if you manage to get him to move behing his bunker. Sounds confusing, but its quite basic.
Also set up barrells behind the bunkers and simply run past them and "mug" them over and over again until every ball you fire hits them - try it left handed (as you should do when snap shooting) and try it from different distances.
There are hundreds of drills you can do in a small space
hope this makes some sense, and helps!