What is the procedure for NRI's to enrol for Aadhaar?

The process is:
An NRI seeking enrolment to visit an Aadhaar enrolment center and submit a requisite form along with valid supporting documents. Enrolment and Update form can also be downloaded from (Enrolment & Update Forms) 

The enrolment operator shall capture the following information during the course of enrolment:

Mandatory Demographic information (Name, Date of Birth, Gender, Address and email)
Optional demographic information (mobile number)
And
Biometric Information (Photo, 10 finger prints, both Iris)
Type of documents presented [valid Indian passport is mandatory as Proof of Identity (PoI)]
Residential Status ( Resided in India for at least 182 days is not applicable for NRI)

After completing the enrolment the operator shall return all the documents along with an acknowledgement slip containing the applicable charges.
List of valid supporting documents is available at (List Of Supporting Documents)  
You can locate the nearest enrolment centre at : (Bhuvan Aadhaar Portal)