This is a little difficult to explain in text, but here goes..
Recline the seat unit to it's lowest position.
On the arm rests of the pram, there is a piece of fabric covering each arm, these are attached via press studs, undo these and fold them backwards onto the seat area.
This will reveal a plastic hook underneath, these are folded down and need to be raised upwards.
Next, get the Ventura carseat and ensure that the handle is in the carrying postition (i.e facing upwards). On the base of the carseat at the back (directly underneath babys head) there is a distinct cut out shape that will fit snugly onto the bumper bar of the pram, place the carseat on top of the pram (so it is positioned on the seat facing you) and push it down gently. If this is the first time that you have done this, the material may be quite plumped up so you may need to push a little harder.
At the base of the carseat handle either side, there is a button (this is used to adjust the handle position) push these buttons down at the same time and move the handle backwards (away from you) until the handle makes an audible 'click', this means that this has now secured onto the plastic hooks that you first exposed on the arms of the pram.
This should now be securely fastened onto the pram.
I would also suggest that you visit this website as per the link below and download the user manual for the 3D so that you have this is picture format in front of you.
I hope this helps.