I've been fortunate enough to officiate four weddings for friends. They have all been different, and all special in their own way. I think that the process divides into two camps: emotional content and logistics.
Emotional content:
- ask them what they want. This usually works best as what they have seen that they liked in ceremonies they have attended.
- there are a ton of ceremonies on various webpages. Download a bunch and parse ideas to run past them
- If they have different backgrounds, make sure you know if there are ceremonies that are important to families that get worked in (break wine glasses, etc)
- I'm not a fan of writing the whole thing for them. I've found that developing a skeleton that they can edit or slash works best.
- The single most amazing thing I've seen in a ceremony was when a part of the ceremony was asking people to stand and give a testimonial to their relationship to the couple. This had a few people who knew ahead of time that they should expect to talk to this to get things rolling. It really tied everyone into the ceremony and emphasized the community aspect. The officiant ends up acting as a bit of a talk show host / MC during this part.
- Understand the tone they want in terms of religion, gooshy stuff, high minded poems, etc.
- Some friends who were rock climbers tied a rewoven figure 8 as part of the ceremony in lieu of a candle lighting. If there are shared interests, modifications like this are pretty neat if they want this type of element in their ceremony.
- have them involved in the writing as much as possible.
- Just ask.
Logistics:
- practice the shit out of it while walking and doing other distracting things.
- know how long they want it to be and work to a time.
- Understand the physical logistics of how the ceremony will unfold: who needs to be where when.
- Know who will have the rings and confirm before the ceremony.
- I pasted ceremonies into old (like really old neat) text books or cloth bound books to hold.
- Print so that it is EASY to read with spacing so that you can easily find your place if you have to look up and down.
- Practice so that you can look around instead of just reading from a page. It's the best seat in the house.
- Know the paperwork requirements before you show up. Most people don't do this all the time and filling out the license/forms can be weird to figure out on the fly when they want to be mingling with guests.
It is a super fun experience and an honor to be asked to do it. Good luck!