Well that's one way I suppose...
But (i'll explain from the top) in general you need to be ahead of the target, moving with the target, be quicker than your target, or be trying to surround something not moving. Once you meet one of those criteria, say you want to surround. You take your army, turn off formation movement (toggled with Alt+F), and hunt down the unit. Now lets say you're trying to surround a target that isn't moving, because these are the easiest. We'll say you're trying to surround AM casting blizz using your dk+5 ghouls (you can do it with 4, but that's hella ****in gosu). He starts casting, just walk up to him using right click, then you want to walk past him enough so that your units generally surround him already. Then once you units are all around him, like you got some ghouls up top of him, below, and some on the side...you use MOVE command and order units to MOVE to the target. This is what initiates the surround. Once surrounded, tell army to attack the target.
Basically in a nut shell you have to "Generally surround the unit" before you actually surround. Think, if you dfidn't "Generally surround the unit" before you did your move+click on the unit, then they would just kind of follow the target and thats not good.
Also key to the surround is SPAMMING move command on to the target...once isn't enough. Once you want to "go in for the surround" you gotta spam the shit outta M+Click because otherwise you won't get him.
NEWays, hope that helps...