I found the instructions for the Heng Long adjustable idler supports to be somewhat bewildering so I once I figured it out I decided to take some photos in the hope that they'd help others if they were stuck.
I should point out I have health problems and my hands are not particularly steady so this may not be the neatest job you've ever seen

Step One:
Use a small Philips-head screwdriver to remove the rear idler and tracks.

Step Two:
Once removed, make a note of the piece of plastic that needs to be drilled out. Everything inside the 'raised hole' needs to be removed, as roughly indicated by the red lines. The 'raised hole' itself needs to be intact, it's just the innards that must be removed.

Step Three:
I began by drilling a couple of extra holes through this plastic to remove some of the excess material. I used a Dremel in conjunction with a large-bore drill bit (which I turned by hand) to remove the remaining plastic. This was a pretty awkward process but with some patience it worked.

Step Four:
This plastic inside the hull needs to be completely removed.

Step Five:
I removed the bulk of it with a hacksaw blade.

Step Six:
This looked truly awful to start with but after some Dremel-action it looked much better - though I'm sure someone with steadier hands could make something many orders of magnitude neater.

Step Seven:
Use the included guide to place the two screw holes necessary for mounting the idler support. I used a 2mm bit to drill a pilot hole through the guide, then drilled that out with a 3mm bit.

Step Eight:
With these mounting holes drilled and the interior of the hull completed and sanded the mounting block can simply be screwed in to place. The blocks are not identical so make sure you fit the right one to the correct side of the hull.

Step Nine:
With the idler block screwed in to position it should look like this from the outside.

Step Ten:
Line up the interior bearing so you can fit the shaft that will support the idler. An allen screw will lock this shaft in to place. The dark-coloured shaft included with the set is used to actually fit the idler, which is pretty self-explanatory.

I hope this helps someone, sorry it's not neater
