to get a full circle grommet into place on an existing tube you need to cut the grommet on a compound angle creating an opening with all 4 edges of the 2 sides ends different. one side of the cut will have a pointed 'arrow head' and the other side will have the 'feathers'. start the strong side of the arrow into the grommet space and continue working the grommet in & around until you get it
make any sense?
the last edge of the grommet through the gap will be the weak side (easiest to get through) of the 'feathers', matching back up to the strong side of the 'arrow head'
get a cheap grommet or 3 from the hardware store and practice cutting them before you decide to jump. my last job i did grommets