To fit the Cabrio (with seat belts only), follow these steps:
Note that you will have three metallic looking blue seatbelt hooks (made of plastic) on the Cabrio. One is at the back to carseat (directly behind the childs head) and also there is one either side of the child (when seated in the carseat), these hooks are used to slip the seat belt into and therefore secure the seat.
Placing the carseat facing the rear windscreen of the car - if this is in the front seat, then ensure that your airbag is disabled if you have one. If this is not possible, then DO NOT use the front seat.
Ensure that the carseat handle is clicked into the 'carrying' position, upright.
Taking the seatbelt in your hands, slip the lapbelt through the two blue hooks which are either side of baby and then click the seatbelt into the buckle. Pull firmly so it's nice and taut, hold onto the carseat when you are pulling the seatbelt.
Whilst taking the slack with one hand, take the shoulder seatbelt and loop through the third blue hook (located on the back of the carseat) and again pull firmly whilst holding onto the carseat.
When you attempt this, practice without the baby in as this is much easier to do.
Finally, check that the buckle of your seatbelt is not resting too high or on an angle of the cabrios plastic shell, as this is considered dangerous and is called 'buckle crunch', for more information on this, please see the link below, which explains with images:
I hope this helps.