Those who win the support of 22 MPs and MEPs, must then get the nominations of 5% of all constituency Labour parties or three Labour affiliates – of which at least two must be trade unions – comprising at least 5% of the fully paid-up affiliate membership.
The freeze date for new members to join and be eligible to vote is January 20.
People will also be able to join as a registered supporter for a fee of £25 provided they are on the electoral register and meet the membership qualification criteria of membership of the party.
Applications to become a registered supporter open at 5pm on January 14 and close at 5pm on January 16.
The second stage of nominations from constituency parties and affiliates then opens on January 15 and runs to February 14.
The ballot of members and registered supporters opens the following week on February 21, closing at midday on Thursday April 2.
The result will be announced on April 4 at a special conference in London.
Clive Lewis has pulled out of the Labour leadership contest after he looked unlikely to secure the 22 MP and MEP nominations needed to make it through to the next round.
The candidates to succeed Jeremy Corbyn had until 2.30pm on Monday to reach the threshold.
Keir Starmer, Rebecca Long-Bailey, Lisa Nandy, Jess Phillips and Emily Thornberry are the remaining candidates.
Lewis said he was standing aside “in the spirit of pluralism, diversity and generosity”.
“For me, this election wasn’t about just the leadership of the Labour Party but about our survival as an engaging and relevant political movement that could win a path to power,” he added.