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.
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.