Baseboards cover the gap between the wall and floor and give a room a finished look. The main skill is cutting clean corners, which is very learnable with a mitre saw and a little practice.
Measure and plan
Measure each wall and plan your cuts to minimize seams, putting any necessary joints in less-visible spots. Buy about 10% extra for mistakes and waste. Account for inside corners (most common) and outside corners (around a wall end).
Cut the corners
Inside corners can be mitred at 45 degrees, but 'coping' one piece to fit against the other gives a tighter joint on out-of-square walls — a skill worth learning. Outside corners are mitred at 45 degrees so they wrap cleanly. Test-fit before nailing.
Fasten and finish
Nail the baseboard into the wall studs (and the bottom plate) with finish nails, keeping it tight to the floor. Fill nail holes and caulk the top edge and corner gaps for a seamless look, then touch up with paint.