I cant say what AB's or DL's like per se as i'm not really into that, but I have had my GF's change me many time, and also my PCA's so from that i can give a bit of advice.
I'll skip the hospital changes and the pca changes.
So:
#1
You need to know how to change someone, if you have NO experiance inchanging a diaper on anyone i'd suggest to just get some baby diapers and practice on a doll or stuffie to get the changing part down, as in how to do everything backwards as it were compared to you doing it to yourself.
Changing someone else is different than doing it yourself, so practice that. and also note that you will likely need some compliance/help for them to lift up to change the diaper as i doubt your going to want to do it like somone that is not able to help at all, aka medical change
#2
This being done between people is very intimate and likely be a very bonding moment and that said, less talking is usually the best thing, as excess talking IMHO makes it awkward.
#3
Depending on if it's sexual at all will make any cleanup a whole different thing, and wether it's going to be a PG rated change or something a bit more.
So, depending on the type of change, make sure you know how to acomplish the change, have the supplies ready and be ready yourself to do the task first.
Then, you need to keep a close eye/in touch with them and see that everything with them is going ok, usually you should be able to tell if they are having fun or something is amiss, if there is anything going awry, just pause for a moment, give a moment for them the catch up or to say anything or even to relax.
If you need to pause and say everything is exposed, gently place the diaper over them in case modesty is an issue, either way it will signal that your pausing for a second and give you an opertunity to see how thing are going.
The biggest thing IMHO is take it slow, dont rush doing the change, and also dont rush the change, look for subtle clues that things are going right or wrong.
This is a very intimate moment and quite often things will overwhelm either of you, just pause or slow down and let your brain and feelings catch up with what your doing.
Also if things are going awry, it may be hard for words to form and come out, you need to look for changes in body language, that may be the only clue you get before things are too far gone to salvage the moments at it were.
Now, as for the ABDL side, i cant personally say to the feelings there, but overall it is a very very very very intimate moment and takes things slowly and do communicate, but mostly watch body language to see how things are going. You will know without words when things are going the right way.
Overall have fun, explore and even if there is an issue and things dont go right the first time, dont become stuborn and stop things from happening again, noone does everything right the first time and noone knows exactly what they want to happen the first time either, so expect that there will be some things that may come up short. Biggest thing is just watch for the "OMG" type of body language, like someone just freezing up, or getting a total blank stare, sometimes things can be triggered that are unexpected and that will end up being bad if you ignore that sign, if it happens.