I'm sure there are different tricks and this is not my specialty, but when needed I use an electric stapler so I can position it, fire and not damage the foam. The hammer staplers run out of room quickly and do cut the foam. In extremely tight places I have stapled to the end I can reach and stuffed some batt insulation into the lower area to hold the baffle in place. The batt insulation also blocks any blown-in insulation from reaching the soffits. If I were to do one today, I would definitely not use fiberglass as the batt, but mineral wool such as Roxul. Much denser and will block wind washing from the soffits.
Bud, Thanks for the suggestion. This is always a troublesome area to work in to begin with.
You might consider switching to a diffrent type of baffle depending on the job - generally an electric stapler and / or foam - I would love to find an electric cap stapler but haven't seen one yet / haven't needed to look that hard for one yet
I am not a big fan of batts as they can get blown into the soffit area & allow for insulation to get past.
I'd suggest buying sturdier plastic ones. Martin Holladay gave a comprehensive list of mfrs. at:
ADO products was making one w/ a wind wash block built in at 90 degree angle - not sure if they still do.
Alternatively, use a hammer tacker and don't swing so hard.