|Making sure that you put the pattern pieces on the grain of the fabric can be both hard, time-consuming and boring. The school-book method is to measure the distance from the grainline marking on the pattern piece to the selvage, but sometimes (i.e. on knits) there is no selvage.
When the grainline is visible on the fabric, for example when there's woven plaid or stripe pattern, this is a fast way to place the pattern pieces: Make sure the grainline marking runs the whole length of the pattern piece, then fold it on the grainline (right sides together), making a sharp crease. Now it's easy to place the pattern piece on grain -- just align the folded edge with the grain and pin the bottom half of the pattern piece. Then unfold and pin the rest.