If you feel you can row with more strength but are not sure what to do, consider the following thoughts on power application. (You can also do many other things, from changing your stroke rate to lifting weights and more to get a better erg score/row stronger, but the following points assume you are rowing regularly and moderately but feel you could reduce your pace in terms of how fit you feel when you row, apart from other tools in your tool box.)
1. Row long: I often shorten my stroke when I am not pulling hard. Be sure you are reaching all the way forward (while still sitting upright/not slumping over/not over-reaching), and pull the arms in firmly to the body to finish the drive. Not reaching out and letting the hands languish over the thighs are sure signs you have shortened up.
2. Catch with the legs: If your leg application follows your beginning the drive with your arms, you have lost out on your primary engines from the catch. As is often said, "hang" on the handle at the catch while beginning your drive motion by pressing the legs down. This does not have to be explosive, but use adequate leg pressure to begin the stroke, which pressure can then accelerate through the drive.
3. Couple legs and back: As you have begun to drive the legs and the knees begin to lower, you are reaching the position with the legs where you have the best leverage. Couple your back/use your back/open your back at that same point/through that same range to achieve maximum acceleration (through the water if you were in a boat - with the added advantage that your blade is closest to perpendicular to the shell so the effort goes into forward movement).
4. Legs finish strong: Believe it or not, it is possible to begin a strong leg drive but then let the legs lag as the body and arms finish. Pay attention to the knees completing the drive firmly and see how much more you feel that in the quads. You can even do sets of 10 'firm leg finishes.'