I shot weddings for ~10 years and loved it. Assuming you have the technical skills, I would emphasize just a few things:

1. Arrange for a second shooter who will help with rounding guests up and posing
2. Redundancy is your friend - have multiple cameras, cards, flash, batteries etc.
3. Minimize lens changes by using a moderate wide to tele lens (something like a 24-120)
4. Aside from the standard pictures, ask the couple if there are any special shots they want.
5. Enjoy yourself - weddings are happy events.

I'm attaching an outdoor shot from one of my early weddings.

OK, against my better judgement, I have agreed to shoot a wedding for my cousin’s son’s wedding. Please don’t tell me it’s a really bad idea and that I should offer to pay for a professional photographer. I know you all mean well. I’ve already done that. As a bit of background:
I recently took a series of photos at another wedding (as a guest … they also had a professional photographer) and sent them out to a number of relatives. This groom to be (this August), his fiancée, and his parents looked at the photos and called me and begged me to shoot their wedding. I said no, no, no and downplayed any expectations, but to no avail. They loved the photos that I did as a guest at another wedding, so I just don’t see a way out of this without jeopardizing our relationship. No, I’ve not shot a wedding before. I’m mostly a landscape and street photographer.
So, I’m going to go ahead with this, and here is where I would appreciate any help and insight you could provide:
I currently own a Fuji X100V (fixed 35mm FF equiv, 26mp camera) and a Fuji X-T2 (24mm MILC camera) with plenty of zoom and prime lenses. The X100V has a leaf shutter, enabling me to fill flash up to 1/4,000 of a second. I have just a little experience using flash outdoors or indoors.
The wedding will be at my cousin's house (with a huge backyard). The ceremony will take place in the afternoon in the backyard, and reception will be in the backyard and also inside the house. Probably around 40-50 people. Lots of young adults and laid back relatives.
What I’m trying to figure out is, which camera do I use? Do I use both? What focal lengths? All the photos that I took at the other wedding as a guest was with my X100V, using fill flash with the built-in camera flash. Do I use one camera for outside and one camera for inside the house? The X100V has a built in digital teleconverter for 50mm and 70mm (in addition to the 35mm) which I can easily change. Which flash unit (if any) should I buy? I’m looking at Nissin and Godox TTL flashes (and my head is spinning). I realize if I use a flash I will have to practice quite a bit for it to become second nature to me (I have 4 months before the wedding). I plan on shooting in RAW + Jpeg, and use PSE and Luminar for post processing.
As you can see, I have some decisions to make before I start practicing. Any help/advice would be greatly appreciated. Thank you in advance!

1. BRING BOTH CAMERAS! God forbid you bring only one and it fails, and swapping cameras is easier than swapping out lenses. Have backup memory cards and batteries.
2. Familiarize yourself with the venue(s).
3. Get an assistant. Preferably a 2nd shooter with whom you can swap cameras when you're not working together.
4. Ask client which particular shots are important/mandatory. Keep a list, preferably in workflow order.
5. Ask for a friend/family member (not immediate, they will be too involved) to assist in wrangling, someone who knows everyone by name, and can handle them if they get unruly.
6. They liked your other work. Study what you did: lenses, flash/no flash, posed, candid, etc.
7. Practice with whichever flash you decide upon. Learn how to bounce, when and when not to, and if/when you need to use direct. Consider a diffuser.
8. BE PREPARED. Expect the unexpected. Outdoors you have no control over the environment.
9. Keep an eye on the clock. Know when cocktails, dinner start. You will not have the freedom you did when you were not "the photographer".

I've not read all 11 pages of prior advice. I'm sure I've overlooked other good suggestions. Best of luck! You'll find a new appreciation for the cost of a great wedding photographer, if nothing else. You can pull it off, but it's a LOT of work.