All of the information you need including links to the forms is below. However, just to give you the process briefly:
You will complete your return, fill out the from W-7, and send that along with the required proof of ID and other documentation. The IRS will then process the return and assign your husband an ITIN.
Some things to keep in mind:
-Do NOT mail original documents for the proof of ID. You will not get them back. Mail copies
-You will be sending your return along with the attached ITIN application and documentation to a special address.
https://ttlc.intuit.com/replies/3300174
This link is to the IRS page on ITIN information, along with the address you will send everything to: https://www.irs.gov/Individuals/General-ITIN-Information