To prevent your floor from sagging and being damaged, you may need to make a floor joist repair if the joist is broken. This should be done as soon as possible after you find a broken joist to avoid damage.
Step One: Cut the Board
You will need to cut the board at least 6 inches longer on each side of the break to give it proper support. Measure the broken area and cut the board accordingly. Then, using the drill, drill two holes in each end of the board about 2 inches from the end the size of the bolt that you have purchased. Be sure the bolts are long enough to go through this board and also through the broken floor joist board.
Step Two: Install the Board
Use the new board to measure where the holes for the bolts should be on the existing floor joist and mark them, then drill. If the broken joist has shifted a bit, you will first need to install one side of the brace board you have made by bolting it onto the old joist and securing with nuts. Then have someone push the old boards until they are even while you bolt in the other side of the new brace and tighten.