Just hold the cap, turn the key until the grip lugs retract, then push the cap on in place with the little arrow pointing outward, then turn the key back to the original position, then withdraw the key.
I've always put mine on with the arrow pointing in, with two tangs it fits either way.
When I remove the fuel cap on my 650, I put it on top of the pump (with the key still inserted). Then I can't drive away without the cap.
And I also leave the key in the cap until I'm done.
I have always left the key in the cap when filling and put it on the passenger seating area. Not going anywhere till its back on.
'03 Coast Cyan Atlantic 500
'14 White MyRoad 700i
'09 Mirage White Burgman 650, Yoshi Carbon/Stainless Exhaust, Bazzaz Z-Scoot Fuel Control
'06 Oort Gray Burgman 650 rebuilt/upgraded CVT
'06 Oort Gray Burgman 650 another parts bike or project, CVT int failure
'05 Sonic Silver Burgman 650 body damaged, stripped down/rebuilding back up on hold
'03 Space Blue Burgman 650, rebuilt/upgraded CVT with Polish adapter
'03 Space Blue Burgman 650 parts bike, failed CVT splines/brg