As far as actually getting something into astral space is concerned, it really doesn't matter. Have a thought...any thought you like...with sufficient emotion behind it, and you can be sure that that is now an astral construct.
The reason why you want to get specific, is because typically you don't want servitors to be one note thoughts. You'll usually want to give them various different characteristics; an emotional profile very often, one or two abilities, (you don't want to go beyond one or two; they lose cohesion and focus, and they start requiring far too much energy) a statue or something else physical to keep it in, a means of summoning it, and a means of feeding it, generally.
http://www.philhine.org.uk/writings/sp_functional.html Here's one example, written about by Phil Hine. I actually still summon this spirit when I'm in supermarket queues; he works well, and is very friendly. Lines start moving much more quickly after I've summoned him.
Another point I'd make, is to realise that when you've first created a servitor, they usually don't have a lot of energy straight out of the gate. So the biggest issue is that you're going to need to feed them up for probably 3-4 days before you can expect them to do anything meaningful.
As a final suggestion; never make them nasty. A bad tempered or vindictive servitor, is a servitor who is just as likely to turn on you as anyone else; and even if they do only target other people offensively, you're looking at karmic problems down the line, because you are karmically credited for everything the servitor does. Whenever I create a servitor now, the last element of the initial creation process for me, is to voice (and strongly mean) the benediction Optimus Prime gave, when he asked Vector Sigma to create the Aerialbots.
"Give this being a personality worthy of She who created me. Let them think for themselves, to grow in knowledge and wisdom, and let them always value freedom and life--wherever they find it."