Fill in the Gaps
Be sensitive to the way the ends fit together. One end has a tongue and the other end has a groove -- this is called end matched. Make sure to always cut the wall end of the wood so that you do not cut off the groove that fits to the tongue. If that happens, that would result in a pretty big gap. Find a piece and lay it alongside the hole and flip it over. Make sure when you make the mark to cut off the wall side, not the room side. When you make the mark, butt it up against the baseboard and then mark at the end of that tongue. That will leave a 3/8" gap for expansion and contraction when installing the piece.
Note: Before nailing, make sure to put at least two nails in every board. The rule of thumb is to place a nail every 10" to 12".