Carefully. Make sure you have a back-up tank in case the two do not get along.
You might try putting the frog within sight but out of reach of the betta first to check for an aggressive response. When I introduced my frogs to my betta, I moved the fish into the frog tank rather than the reverse to avoid territorial behavior and watched VERY carefully, ready to remove him if he seriously went after anybody. After the first half hour I checked on them frequently and looked everyone over for bite marks or injuries each time.
It ended up not working out, not because of aggression, but because the betta was too nervous and scared of the frogs and wasn't doing well. So they now both happily have their own tanks.
Robert Falcon Scott - Red salamander HM plakat
Passed on- Friedrich "Freedy"
Four African Dwarf Frogs - Dorkus, Sporkus, Baron(ess), and Bubbles!