There are several things you can do and it depends how expensive and/or is it a favorite rod.
I will assume that this is a 1 piece rod with synthetic grips.
If it just turns around and does not wobble, you can align it properly to the guides with tape holding it in place. Drill 2 holes completely thru from the sides. Used 2 bolts and nuts with lock washers to anchor it. Wrap electricians tape around the seat where the nuts and bolts are located. Go fish!
You can try to drill several holes just thru the sides of the seat(not into the blank). Then using a syringe squirt 5 minute epoxy into the holes. Make sure a lot of the epoxy gets into the seat area. Align the seat to the guides. Let dry overnight.
Now we get to the expensive way, if you pay to have it done. Remove the butt cap (cut it off). Cut off the synthetic grip with a knife, serated edge works ok. Slide seat off. Sand butt grip area as smoothe as you can. Do not cut or sand into the blank itself. Install the seat using a semi paste 2 part epoxy. Align properly to guides. Allow to dry 24 hours. Install a new butt grip. This can be tricky. Install butt cap. Use semi paste epoxy for last two components. If you use this method contact me for instructions on how to install a synthetic from the butt.
Another way. Cut off butt cap. Tape seat firmly to each grip in its proper position. You don't want it to move. Using a hacksaw cut the seat and blank in half (at mid length).Using a small rat file rough up inside each piece of blank about 5". Spray 91%isopropyl alcohol into the area just filed to clean area. Make sure the cut is made on the bias (not at right angles). Doing this will make it possible to align the 2 pieces of blank correctly when assembling. Now you have to find a piece of blank to slide in from the butt end. We are going to make a sleeve that fits tight inside the blank. It has to fit the butt end and also the top piece. Remember a taper is involved with the fit. The sleeve has to extend about 4"s above and below the cut, in all total 8"s. Sand entire sleeve using a scotch pad and clean with 91% isopropyl alcohol. Epoxy the sleeve to the butt end. Let dry 24 hours. If the arbors that positioned the reelseat are still ok proceed. If not, you have to dry install the seat to the blank(both pieces held in place)by building arbors (rings) using 1/2" masking tape.Leave 1/2" or more between arbors for epoxy to fill in. You want the seat to slide over the tape arbors snugly, not tight. You might have to rotate the seat for final alignment. Smear plenty of epoxy into top part of blank. Coat bottom part also. Slide reel seat onto bottom part. Load up interior of seat so that there are no air gaps. Slide top part down onto butt sleeve. Align the cuts that were made on the bias. Make sure the cuts align and stay together. Slide the reel seat to align with the guides. Double check the alignment to the bias cuts and the seat alignmeny. Clean off all the epoxy with 91% isopropyl alcohol and paper towels. Keep cleaning till all exterior epoxy is removed. Let sit for several days. Install new butt cap.