You slide it in from above.It is RIDICULOUSLY difficult to get the intake assembly to fit in the space given, especially as the filter ages and warps - if there is a secret to it I haven't found it.
When you have cleaned an AquaClear HOB filter and put it back on the tank, slide the intake tube into position from above, fill the filter with tank water, then turn it on. The filter will suck the water up and spit out some air, then it will run.
If you have concerns about the size of the gaps in the intake strainer, put a round/ cylindrical sponge (from an internal power filter) over the intake strainer screen.