Well, because I am a clown of course. Just kidding, Shaco is now top 1 support in the game. Of course that is also because only few people tried to play it like this. But statistics do not lie - Shaco has great potential, and can be surely used as a support.

Basically there are two options for you:

1. Make your jungler's life better by helping him out on blue/gromp or red/krugs. Simply put two boxes here and there and make him finish it much faster than usual. Your boxes will deal enormous damage and also tank the damage. After that, your jungler will be far ahead, and that is important, especially at the very beginning of the game

2. Put some of your boxes just like in the screenshots below - depends what side you start.
If you start red side

If you start blue side
sometimes the enemy will activate some of the boxes even before killing the first wave. It is very important as they will be left with low HP, losing probably the first wave and giving you the advantage early on.

On lane you should focus on poke, and obviously on securing your back from ganks or unwanted fights. Always remember to get at least one box on the back (preferably two), in a place where it will not be activated by enemy minions (check screen shot below). If it comes to poke, your main weapon is your E combined with arcane comet, but you can also put your boxes (from time to time) right at enemies legs, to keep the pressure (they will have 2 seconds to destroy it, well trained will allow you to keep the enemy failing in farming)