This is Article 50, now well-known after being invoked for the first time by the UK on 29 March in Prime Minister May’s letter to the President of the European Council. Article 50 envisages four phases. The first one, already completed, requires the outgoing member state to notify the European Council of its intention to withdraw from the European Union.
The other steps are as follows: the Commission is given a mandate to negotiate the withdrawal agreement according to the guidelines unanimously established by the Council; the Commission conducts the negotiations on the withdrawal agreement; and both parties approve the agreement.