Use a tapping block to close the tongue-and-groove joints together or hire a secret nailing tool to push the boards together and nail them.
If you do not have a tapping block then make one from waste wood flooring. Use a four to five inch waste piece of flooring and cut it lengthwise on the tongue side about two inches in. This will give you a block about five inches long and two inches wide, which will fit into the groove of each board.
Ensure that all the tongue and groove joints, including the end joints, are fitted together tightly with no gaps.