Use a stud finder to locate two or more studs in your wall. Measure the space between them. This will tell you the spacing used throughout the home. With this knowledge, you can measure entire walls and mark a nailing pattern. If the drywall has been left high enough, you can even see the studs from floor level, speeding up the process.
Mark the location of the studs on the wall with a pencil so you know where to nail. Starting from the corner of the room, extend your tape measure and use the distance and spacing determined by the stud finder to place your pencil marks.
Double-check your pencil marks occasionally by waving a stud finder over them. If they don't confirm the stud placement, check your measurements before proceeding.
Add additional nails as needed during the installation. Even nails that don't hit a stud will pull in the baseboard somewhat. If you have a small gap, put a nail in the base at that location and try to pull it in.