first drill holes big enough for the bolts to go through. then apply a little glue to the t-nut and hammer it in to the hole from the other side. make sure not to get glue on the thread. then you stick you mount the subwoofer and tightened the bolt.
if you don't hammer it into the wood first, it will fall off. how are you gonna get your hands in there to hold the t-nut in place while you bolt the sub on.