Yeh I started holding troop back in October when it first started.
So first will be your psychometric test which you should be able to book pretty quickly. Then an interview with your careers officer, followed by the dreaded medical. First will be a phone call one where they ask you if you have ever had any injury/disease that's ever existed, then the face to face one if you pass. This is the point which slows most applications up, I recommend taking all your medical history with you otherwise you'll get TMU'd until they receive the information. So for example if you've ever had an inhaler, get a note from your doctor showing when you were last issues one and show that you no longer need it etc. This tripped me up.
Once you pass the medical, that's when you can join holding troop. We meet first Saturday of every month at Wandsworth and usually go through the RMFA. Thankfully reserves don't need to do the PJFT on the dreaded treadmill but instead have to get under 10:30 on the return on a BFT. Once you do, they'll load you onto the next PRMC.