When putting together two baseboards, you need to make a joint between the two. When joining two straight pieces, you can use a 45-degree cut to make the seam appear as small as possible, and you can also use a 45-degree cut when you're trying to meet another piece of baseboard in a corner. Use a mitre box and hand saw to make these cuts.
With the pencil, mark the baseboard where you want the cut to be.
Set the mitre box on the ground and locate the two 45-degree slots in the box, which are labelled. Insert the baseboard in the mitre box, aligning the line you made in step 1 with the 45-degree slot in the box in the direction you require. Verify that the baseboard is vertical in the mitre box, and that the inside edge of the baseboard is flush against one side of the mitre box.
Position the handsaw in the 45-degree slot in the mitre box so that it's above the baseboard. Pull and push the handsaw back and forth in short strokes while holding the baseboard against the mitre box with your other hand. The slot works as a guide for the saw so that you can cut the baseboard accurately. Continue sawing until the baseboard is cut completely.