Car booster seat with H harness

I have daughters nearly 2 and nearly 4. The nearly 2 is about to outgrow her car seat so we decided the cheapest option was for DD2 to get DD1 Maxi Rider car seat and buy a booster for DD1. We decided on the Ifa Secure VArio Kid Max given it has rated the highest in safety testing. We also purchased a H Harness to be used with it, we got the infa secure branded harness as it was recommended you use the same brand as the car seat you get.

Now here is the issue I have, it seems really difficult to get my daughter in and out of the car seat using the H Harness. Do other people find this? Do you have to thread the lap slash seat belt through the loops each time or are we missing something and there is an easier way of getting it over her head without rethreading it?

We bought 2 boosters - 1 for each car, I haven't taken the other out of the wrapper yet coz I'm very close to returning it. All the reviews I've about this booster rave about it but I just don't get it. Maybe they aren't using it with a H harness and have just started with the normal seat belt.

If it's the one I think it is there should be a red clip attached to it. Put her in the seat with the harness, do up seatbelt then use the clip to clip both parts of the seatbelt together about thigh height on door side. This stops the setbelt from retracting every time you undo the belt. Then you just lift the harness over her head.
Hope this makes sense :)