Remember this test can damage equipment and there is also a risk of electric shock from the d.c. voltage produced by your test instrument (usually 500v).
We are testing to ensure that the insulation surrounding the conductors is adequate and that there are no short circuits between conductors. It can be done on a complete installation or on individual circuits.
See Part 6 BS7671 Table 61 for the correct voltage test setting and the minimum insulation resistance permitted. Eg. Test voltage 500v for 230v or 400v installations or circuits with a minimum insulation resistance of 1 M ohm (1000,000 ohms). A combined resistance of 2 M ohm or less for all circuit should be investigated further to find the individual circuit(s) that may be causing the low reading. A point worth remembering is that combined resistances measured in parallel will always be lower that their individual combined readings (multiple resistances in parallel formula).
For a step by step guide have a look in the On-Site Guide or Guidance Note 3. By the way the On-Site Guide section on Inspection and Testing is near enough the same as GN3.
The On-Site Guide is FREE with you 17th Edition online course here at the-Regs.co.uk