There will be quite a bit of information available online about how you take a possession, the types of protection etc. But I think OP is asking more about the planning process that leads up to this?
My experience from working on the fringes of this planning process:
All requests to work in a possession are booked in Network Rail's Possession Planning System (PPS) with details of the location, the team responsible, and the type of work. Because there are lots of different teams with work to do and limited time to do it in, PPS will also collect information to help decide which works are compatible with each other (maybe surveyors measuring the track at the same time as signal technicians checking wiring) and which will clash (such as using a rail-mounted tree-chipper at the same time as using a machine to replace worn rails).
There are then a series of 'T minus' meetings in the run up to the possession, named for how many days or weeks they are before the job (eg: T-8 weeks, T-10 days)... conflicts can be resolved at these. Compatible jobs may be grouped together into worksites under the control of a single Engineering Supervisor. Or a job might be cancelled, requiring another team to take control of the worksite. One team might agree to use a different access than originally planned so that they don't conflict with another workgroup. Possession limits might be extended to include an important junction or shortened to avoid stabled trains.
The biggest, most disruptive works that require possessions outside the rules of the route (such as a three-day long blockade) are booked in at T-40 weeks or even earlier. I think most 'rules of the route' works that take place outside of timetabled running hours (mostly overnight) are booked in around T-13 weeks or T-8 weeks. The nearer it gets to the job, the harder it is to change details, and the higher the chance is that someone else has already planned clashing works that make your works impossible.
Final possession details are agreed the week before, and the details published in the Weekly Operating Notice (WON) which is sent to anyone who needs to have it.
