Look at my build in bragging spot..... I just left the old stringer which after probably 30 years still were like brand new, and I installed new ones right above them. I sanded down to the old stringers, filled the gaps with resin, and then roughed it up with the grinder, and glassed in the two new stringers above them. I installed two stingers but some people install 4 or even more. Just depends on preference.
I assume you don't have any stringers in there now? it's pretty easy, just takes some time, make templates using cardboard so you don't waste wood. Use epoxy resin, not poly, and it will last for many many years. Here is a picture of a nice stringer layout.