The most non-negotiable thing, obviously, is going to be your officiant's ordainment. Your friend must be ordained and their ordainment must be considered legal wherever you are planning to conduct the ceremony. Also if officiants need to be registered in that geographical area, that has to happen in advance. The Universal Life Church has a great website for checking out the various laws, rules, and red tape in your state.
You probably hear from people all the time that it's free to get ordained online, and really it's true! The part I cannot help you with, however, is with whom, or through which church your officiant decides to get ordained. This decision is ultimately personal. The Universal Life Church does a fantastic job of integrating all faiths. Myself, an Atheist, opted to get my ordainment through the Church of Spiritual Humanism.
Depending on the state you're getting married in, your officiant may or may not need to obtain what's called a "Letter of Good Standing" from the church they were ordained through. For a small fee, the church will send a document on official letterhead (possibly notarized) stating that your officiant is a minister in good standing with their organization, and is approved by the church to conduct church matters.
FILLING OUT THE LICENSE
This gets its own section because, while not difficult, it can be fiddly, and there are a few points I'd like to make regarding it.
Be sure you bring three things with you to the ceremony: a proper envelope (security print on the inside is a nice touch, as you'll be sending social security numbers through the mail), stamps, and a good pen (USE BLACK INK.)
When you get the marriage license, take time to give it a good look over and understand what information you'll need your officiant to fill in. Licenses look different and require different information in every state, so take your time! Even here in New York there are different licenses- one for New York City, and one for the rest of the state- and they're completely different! The officiant will always sign, the witnesses will always sign, sometimes the license will ask for the residential addresses of the officiant or witnesses, sometimes the bride and groom have to sign, sometimes you need to fill in the county or location of the ceremony, etc. If it would help to go over the license with your officiant prior to the ceremony time, please do! Always better safe than sorry.
Once the license is properly, thoroughly filled out (good job!) your officiant will need to mail it back to the city or town clerk that it came from. There is usually a very tight timeframe in which to do this (where I come from it's five days). That's why I said to bring the envelope and stamps to the wedding, because you'll want your officiant to mail that sucker on their way home. It took me six years of officiating and madly dashing for the mail box on day five every time before I realized I should really keep stamps and envelopes in my car.
BUILDING A CEREMONY
I will start in the most important place: don't leave anything out.
I attended a wedding once where the officiant was a friend of the couple, and she, my hand to god, forgot to do the ring exchange. I, being an officiant, was only kind of horrified. The couple kissed and then kind of looked at one another and then looked at her and were like, "Uh... the rings?"
Before anything else, make sure the ceremony has the following things:
Everything else is gravy, all the preamble, the readings, the unity thingies, postamble, it's just dressing for those four items above. Don't forget to do them! That being said, I know, and you know, that you want the dressing. A ceremony is a reflection of a couple's spiritual center, even if they aren't particularly spiritual.
It can feel a little crazy not knowing where to begin, not knowing how to go about writing a ceremony script. Where do you even start? Your best bet is to search online for basic examples of ceremony scripts (specifying 'religious' or 'secular' or 'modern' etc.), readings, and unity rituals. Then, start piecing together the parts you like. Remember not to make it too long- twenty minutes tops is a great rule of thumb.
See a written draft of the ceremony at least a month ahead of time. Whether you work to write it together, or your friend is writing it all themselves, you're going to want to see that script and know exactly what is being said. You want no surprises.
Of course, if you and your officiant aren't sure you trust yourselves to write the ceremony, or simply don't want to try, many professional officiants offer ceremony writing services. I, for example, will ask you all the same questions I ask my wedding clients, write a ceremony from scratch that your officiant can then perform for you, as well as offering myself to your officiant for any advice they need about getting ordained, filling out the license, being ready for anything, and even just assuaging their performance anxiety. Because believe me, I've been there!
Wait, you just said my officiant might have performance anxiety. I don't want that.
It's okay. There's something about a wedding that makes everyone nervous. Even my heart still flutters around the moment I step up to the altar. If your officiant is feeling nervous, remember to tell them to focus on you, because ultimately, the couple standing at the altar are the only two people that ceremony is for, even if there is an entire congregation listening in. Your officiant knows you and loves you, and so they should consider the altar space sacred ground that you all get to stand safely in. The congregation is outside of that space, and don't require acknowledgement or engagement unless the officiant is comfortable. Tell your officiant to imagine they're reading you a bed time story. The story is for you, the moment is for you, and there's nothing to be afraid of.
Do a rehearsal. Don't think you need one? Do it anyway. For a first timer this is an imperative and it's going to let you guys sleep easy knowing that no one is going to be on their toes on ceremony day. For a new officiant, the rehearsal is equivalent to a Xanax when it comes to calming nerves, walking through the space, knowing where they have to be and when, who they should check in with when they arrive for the ceremony, where the DJ will be set up in case they need a microphone, who will have the rings, the list goes on!
An extra bonus thing you can do (which I love to do,) is to fill out the license at the rehearsal. This takes the pressure off your officiant on ceremony day, as they won't have to chase you and the witnesses around a crowded party atmosphere! Just write the date in as the correct ceremony date (not the rehearsal date) and be sure that the officiant mails it on the ceremony day.
Really, ultimately, that's it. Your friend or family member is bestowing on you a great gift, a beautiful favor. Given the right preparation and knowledge, everything should go just fine- and if it doesn't go just fine, (if, say, your officiant forgets the ring exchange?) Well, laugh about it and move on. You only get one wedding day, and regardless of the mishaps that mis-happen, it's going to be one amazing party that you remember for the rest of your life. And once those rings are on your fingers, you'll forget that anyone ever forgot to ask you to put them there in the first place.