Carefully. The old doubles, especially underlevers, are not known as strong actions. But, there are a few ways to do the conversion. The first is to line the 12 guage barrels with whatever rifled liners you want to use. Then chamber and test fire. Regualting the barrels is a bit harder but it can be done. Lining the barrels results in a heavier rifle.
Another idea is to build stub barrels for the action. Then you need to add ribs and such and regualting is a lot harder.
Extractors need to be rebuilt and sights added but it can be done.
I would suggest contacting whoever is going to do your build and ask them what they will do.