+1 for that usual good advice.
They hook you in with the promise of drinnking beer in 21 days, then you drink it... :sick:
Not that kits are all cr@p. I brew one a week! The instructions tell you roughly what to put in it (although you generally want to replace the sugar with spraymalt), the hot water to cold water ratio (6 pints hot to... well the rest cold) seems to get you bang on pitching temperature...
...at that point you are dealing with a living thing which will do what it's going to do the way that it wants to do it! Living things don't follow instructions...
So, like a shepherd, you just tend your yeasty flock. Leave them to settle in and party on down with all that sugary wort then once the party's over (steady gravity readings two days apart and inline for the style of beer, from 1.012 for two can ales to 1.006 for lagers and lighter beers), let them get over their hangovers and clean up the house - should take them two or three more days.
Once they've straightened everything out, they'll rest leaving you with nice clear beer that you can bottle. Then leave it as long as you can possibly resist drinking it. A couple of weeks to carbonate and at least another couple of weeks somewhere cool.