the basic principle is that you make a 1.040 DME batch that you pitch the yeast into. The size of that batch is determined by how big your beer is. You then either use a stir plate, or swirl the starter every once in awhile, to keep it aerated. The starter is done fermenting when you see a big sediment layer on the bottom and relatively clear "beer" taking up much of the volume.
As for "is it too late"...no, as soon as you take out your yeast from the fridge and raise it to room temp....it's now a viable, pitchable yeast. The main thing that you're doing when you put a batch of yeast in the fridge is that you're making those yeasties go to sleep. If you've gotten a slap pack to swell up, that just means you've gotten some yeast to multiply. Those new yeasts still will either go to sleep or set forth and binge depending on what temperature and food they have.