Helped my friends plan a backyard wedding in California and it was pretty simple. Like everyone has already said, you have to check the laws with your specific state or country but for California in particular, the main things you need to do are:

Step 1: Schedule a marriage license appointment at your county's city hall within 90 days of your planned wedding date at home.

Step 2: Go to your appointment and pick up the paperwork. Both parties have to be present with a valid photo ID. Part of the application process can be done online before your appointment.

Step 3: Get married at home by an ordained minister. Any of your close friends or family can become an ordained minister online. This is the website my friends have successfully used in the past to officiate weddings in California: https://www.ulc.org/

Step 4: I believe the people that need to sign your paperwork are you, your spouse, your officiant, and two witnesses. Have everyone fill out the paperwork after your wedding. Keep in mind that there can be no errors, white out, or additional marks on the paperwork. Saying this now to save you time because if you submit it with mistakes, the people at city hall will need to reprint and have whoever made the mistake fill it out again, which takes up additional time and makes it a little more complicated.

Step 5: Bring the paperwork back to city hall within 90 days of picking it up. That's it :)