You're *probably* okay to decant most of the liquid and pitch the yeast slurry after only a few days. I'd probably still make a starter, but that's just me.
To make a starter:
Use dry malt extract (the lightest you have on hand) to make a 1.030-1.040 wort. No hops necessary.
Easy to remember formula is 1 gram of DME for each 10 ml of water.
So if you want to make a 1 liter starter (1000 ml), you need a liter of water and 100 grams of DME.
Bring the water to a boil, take it off the heat, add the DME and a quarter tsp or so of yeast nutrient, stir, bring back to a boil and boil it for 10 minutes.
Cool it just like any beer wort, to pitching temp (I'd take it down to no higher than 75F, preferably 70F before pitching).
Sanitize your starter "fermenter"...I use a growler for my yeast starters.
Put the wort into the growler, pitch the yeast, put a piece of sanitized foil over the top (don't put a rubber band on it or anything).
Swirl it vigorously for a couple minutes, then put it somewhere between 70F-75F and let it ferment.
Pick it up and swirl it a bit every time you pass by or otherwise think about it. Every couple hours is optimal, but I never get around to it quite that often.
Optimally, pitch the whole thing into your batch of beer at high krauesen, usually about 18 hours from when you pitched the yeast into the starter.
If you want, and some people prefer it this way, let the starter ferment out completely, at least 24 hours if not 36 or 48, then pop it in the fridge at least overnight, then decant off most of the liquid, swirl up the slurry, then pitch it like any vial or smack pack of yeast.
More info here:
http://www.mrmalty.com/pitching.php
and here:
http://www.mrmalty.com/calc/calc.html