A funeral ceremony needs to be written down from beginning to end. You could try doing it all just from notes, but that might be living dangerously.
By writing it all down you can keep an eye on timings. You’ll want to use the time you have in the most profitable way, and to allocate more of it to some parts of the ceremony than to others.
You can time your script using this measurement: 100 words = 1 minute. Remember, if you are using a crematorium chapel, the worst thing you can do is go on too long and keep the next funeral waiting.
Writing everything down also means that you can share what’s been written with other people, invite their suggestions or input, and end up with something everyone agrees is just right.
If everything is written down and someone at the funeral finds they cannot carry on, somebody else can come up and take over. This is what families and friends do for each other.