Combination of contact cement and screwed-on strips. Be sure that the contact cement layers are thin (but still viable) and even, as a "blob" of glue can transmit light! The screwed on strips can help mitigate this light-piping issue, as well as help to ensure that things don't get pulled apart as the bellows is stressed over time, by various movements, expansions/contractions, and environmental conditions.

The aforementioned strips can be omitted...but if so, be sure to give the cement a week or so to cure more fully before putting any stress on the bellows. Sometimes, if a "strip-less" bellows begins to separate from one or both frames, the strips can (if design/mounting permits) be added at that point, with it being a 50/50 proposition as to whether you'll want to pull the bellows off and re-glue first. My advice is to first pull gently to test the old glue for resistance, because pulling too hard can tear the fabric and make a mess!