In order to obtain the "International Doctorate" mention, a minimum stay of three months must be completedand must be carried out outside the Spanish state in a higher education institution or research center (host or destination institution), where the doctoral student is studying or carrying out research work directly related to his/her thesis. The stay can be carried out in one or several periods, with a minimum duration of one month each.

https://www.ehu.eus/en/web/doktorego-eskola/outgoing-stays

The procedure is as follows:

1. The academic committee of the doctoral program authorizes the stay.
2. The procedures to be followed for outgoing stays (UPV/EHU origin) can be found in flowcharts 208.2.1.2 (Erasmus mobility) and 208.2.2.2.2 (other stays).
3. At the end of the stay, the doctoral student must request a certificate from the host institution (Certification of the foreign organization of the stay) signed by the person responsible for the stay.
4. The doctoral student sends the certificate of stay to the DOKe.
5. The doctoral student must complete a report of activities related to his/her stay at the host institution, which will be included in the Activity Document through GAUR.
6. The doctoral student will write at least the summary and conclusions of the doctoral thesis in one of the usual languages for scientific communication in his/her field of knowledge, other than any of the official or co-official languages of Spain. This rule will not apply when the stays, reports and experts come from a Spanish-speaking country (article 25.2 of the Regulations Governing the Management of Doctoral Studies)
7. Prior to the presentation of the doctoral thesis in the department in which the doctoral student is enrolled, the director or directors must obtain at least two favourable reports on the doctoral thesis issued bydoctoral experts who are regular members of two different institutions of higher education or foreign research institutes (article 25.3 of Regulations Governing the Management of Doctoral Studies). These reports must include a brief CV of these experts.
8. The doctoral student follows the authorizationprocedure  that applies to any doctoral thesis (with or without mentions) up to the submission to the department in which he/she is enrolled (article 13 of the Regulations for the Management of Doctoral Studies) and requests the mention "International Doctorate" (form 18).
9. In addition to form 18, the favourable reports referred to in step 5 and the certificates of stay issued by the foreign organization (step 2) are submitted to the department and all the documentation is submitted to the Postgraduate Studies Service to be processed  by the Postgraduate Commission of the UPV/EHU.
10. The Department Council prepares a proposal for theexamination committee.that is submitted to the Postgraduate Commission. In case the "International Doctorate" mention is applied for, the composition of the examination committee must meet additional conditions. Thus, for tribunals of five or three members, at least one of the full members and one of the alternates must belong to a foreign university or higher education or research organization. The person who has certified the stay may not be a member of the panel, unless there is another international member on the panel.
11. The postgraduate commission authorizes the defence of the doctoral thesis (including the examination committee).

After the defence of the doctoral thesis, the secretary of the examination committee will include the languages of defence and their percentage in the grade report.