You should replace the ducted vacuum bag when it is full, which will depend on how regularly you use the vacuum system. With the GX series ducted vacuum models, the intelligent monitor system indicates the right time to change the filter-bag.
Start by removing the dust collector of the ducted vacuum.*
Pull out, seal and toss the old bag.
Position the new filter-bag ensuring the notches on the adapter are lined up with the bag's opening.
Have the bag inserted completely (up to the retention ring). Then, rotate 20º to make sure it stays in place.
Reinstall the dust collector of the ducted vacuum.
*Procedure is a little different if you own a GS110 ducted vacuum. Instead of removing the dust collector, remove the power unit's top lid.
You should also replace the disc-shaped filter (between motor and dust bag) after you have changed the filter-bag three (3) times.
Make sure you only use Cyclo Vac replacement filter-bags. Failure to do so could void the warranty on your ducted vacuum.