Check out these two videos. One is for Unit Leaders, like Scoutmasters and one is for Merit Badge Councilors. They show all the details of how to use virtual blue cards inside the Scoutbook application.
https://help.scoutbook.com/knowledge-base/how-do-i-assign-merit-badge-counselors-to-a-scout/
For those who are challenged to use Scoutbook, either because the Troop has not fully implement Scoutbook or because the Merit Badge Counselor is not fully accustomed to Scoutbook, an alternative, while not perfect would be to do the following.
Have the Scoutmaster sign a Blue Card for the Scout (filling in the Counselor’s Name and signing the Scoutmaster’s approval). Scan or take a picture of both sides. Send to the Counselor and Scout. When complete, the counselor prints out the Scan/photo from the Scoutmaster, signs and dates the Blue Card, and send a new scan/photo of the completed card to the Scout and Scoutmaster in an e-mail certifying completion of the Merit Badge on a specific day. The physical Blue card can be completed after the shelter-in-place but the electronic image of the signed blue-card should suffice as evidence of completion for advancement..