You need to be:
1. Competent
Based on proposer and seconder comments plus feedback from other members who have accompanied you climbing / mountaineering.
2. Suitable/Clubbable
Based on proposer and seconder comments plus feedback from members who have stayed in a hut with you or met you on a meet.
3. Demonstrate Club involvement
You will normally demonstrate this during your Associate Membership. It may include meet attendance, including maintenance meets, co-ordinating a meet, helping with guidebooks or getting involved in other club activities.
4. Start the membership application process online.