I gave up putting my matrix seat in the lie flat position in the car after 3 weeks as it was such a pain to fit and just put it flat on the chassis.
Anyway, for some assistance.
Click in the seat belt as normal and then put the car seat on the belt, thread the lap belt under the car seat ensuring that it has caught behind the two 'levers' IYSWIM. Then pull tight so it is snug. Then thread the upper part of the seat belt round the back of the seat ensuring it is caught between the two 'holders' and then pull down the handle into the auto position - does become second nature with practice. Hope this helps - if you are still confused then post again and I will try to be a bit clearer.