SA questions - please advise!

Hello! We just finished our second round of unsuccessful Clomid. Before our 3rd and final round, my OB suggested my husband get a SA done. Where do I even start? Does he go to his GP who orders the lab? Does he need a referral to a urologist, etc? Anything else I should know? I’m clueless when it comes to the male spectrum of things. I am otherwise healthy. We conceived our daughter on the 1st try no problems but have been TTC for months now with no luck. Thanks in advance for any advice!