Essentially, it's a piece of 70mm x 45mm scrap wood cut to length and profiled to fit in place of the drip tray, with a hole drilled exactly under the centre of the group. Into said hole fits a threaded rod with a plastic end cap (also a piece of scrap found in the shed [not sure what it was originally]) and three nuts.
The top two nuts are tightened together to provide a bracing point and the third nut is threaded on loosely below. The rod then sits in the hole with a washer under the bottom nut.
Slotted back under the group, with one spanner or wrench on the top nuts, the bottom nut can be threaded with a second wrench/spanner down on to the washer so raising the rod up to support the piston.
Then it's a simple matter of removing the lever pins and lowering the piston by turning the bottom nut the other way in a controlled fashion. Replacement, as they say in all the best manuals, is achieved by reversing the procedure.
And IT WORKS! Presumably also would on other machines, mutatis mutandis.