First look over the pattern layout in the pattern instructions and take a moment to recheck which pieces you'll need and how many times you cut each. Now lay down your first piece according to the instructions. Don't pin it yet!
Find the grainline on the pattern piece. Unless the piece is on the fold (we'll get to that in a moment) it's the long line with an arrow at either end. Measure from it to the selvage or fold (whichever is closer) at several points. Is it the same distance from the selvage/fold at each point? If not (and it's probably not) then adjust it until it is. Now put two pins in along the grainline. If you skip this step you're likely to cut the piece off-grain and that can later cause the piece to hang wrong and try and twist around your body. If you've ever had a garment whose side seam wanted to be in the front then you know what a pain it is. And no, there's no way to really make an off grain piece behave like it should. So don't cut off grain!
Now smooth out the pattern with your hand, making sure it and the fabric are flat. Pin around the edges of the fabric, putting the pins parallel to the cutting line and about 1/2" in. I put pins ever 6 or 7 inches on long straight edges and every few inches on more curved edges.
If you look at the picture you'll notice how I pin the corners. Rather than pinning parallel there I put them at an angle. It seems to hold the fabric better. Make sure all your corners have pins as this will help you a lot when you get to cutting.
Excursus: Pinning patterns that go on a fold.
Placing and pinning a pattern piece that goes on a fold is, in some ways, easier than other, non-fold, pieces. Pieces that go on a fold are indicated by, instead of a grain line with arrows on both ends (or on one end in some cases), a line about 1 inch in from one side with arrows pointing to the edge. That's the side you'll want to put on the fold. I just lay it down and adjust it until the pattern is right on the fold all the way down and then pin it. You can measure the distance from fold to grainline if you really want (I usually don't) but if you just line up the pattern edge with the fold then you should be fine. And that's it! Easy!
Repeat this for the rest of the pieces or at least as many as you can fit onto your cutting surface. Now you're ready to cut!