Cut the baseboards to the length you will need using a miter box and saw. For pieces that will meet in a corner, position the baseboard in the miter box so it is set at a 45-degree angle.
Paint or stain the baseboards and let them dry before you install them to make this step easier to do.
Locate the wall studs behind the wall with a stud finder, and mark them with a pencil.
Install the first baseboard by starting at an outside corner. Attach the baseboard to the wall where the wall studs are marked by hammering a finishing nail in at each stud location. Hammer the nails into the baseboard until the nail head is flush with the baseboard.
Apply wood glue at the corner joints to make them more secure.
Put a nail set on the head of the nail and tap the end of the nail set to drive the nail below the surface. Fill nail holes with a wax putty stick.