1. Get elevated to the peerage on the recommendation of the Prime Minister. The great majority of Lords appointments are made in this way, and this category of appointments has several subcategories:
a. Party political peers appointed in the dissolution or resignation honours lists or in periodic lists of new working peers.
b. Individuals appointed specifically to enable them to serve in the Government (so-called GOATs).
c. Non-political appointments of senior public office-holders such as Cabinet secretaries and generals.
2. Apply for appointment as an independent Crossbench peer to the House of Lords Appointments Commission. The Commission was set up in 2000, and also has the job of scrutinising political appointments to the House.
3. Win a by-election for one of the 92 seats reserved for hereditary peers.
4. Become a senior bishop of the Church of England.
For further information, see this publication by UCL's Constitution Unit.