Remove the rear cap and unscrew the bolt or allen head and remove. Depending on your choice of stock, it may include a new longer bolt or you will have to provide one.
In my experience, most stocks will require that you provide the longer bolt for attaching the stock. If you have a Lowes or similar nearby, just take the shorter bolt there and get one the same dia. that is as long as you think it needs to be.
Thoms, when I removed the bolt from my Defender (1985 model with pg never removed before) I found that the factory had used some form of locktite on the threads. The reciever is aluminum and soft. I suggest you spray a lubricant into the base first and hope that it will soak into the threads. The threads in my reciever were damaged upon removal of the bolt. You can break down the sg and spray from inside the reciever.