How do I prepare my mentally disabled adult daughter that she will be moving into a group home?

For my sister, she would visit the new place, get to know the staff. When she moved, all her personal favorite things went with her. We then make NO contact for two weeks while they settle her in. That is the only way she would change her focus to her staff and interact with her roommates. Taking her back home for a weekend only leads to confusion and agitation.
It's like taking your child to Kindergarten-- you can't go in there with them because they need to be independent. Regardless of ability.

LindaSusan, you will need the conservator and POA to arrange this for your daughter. Are they on board?

Thank you all for these great answers, even the funny one from "captain"!

I was thinking the same as Windy, if she could spend time there during the day, interact with the staff and perhaps make a few new friends the prospect of a "sleep over" may be more exciting instead of fraught with fear. And of course you need to go in with a positive attitude as well (fake it if you have to) because she will undoubtedly pick up on your anxiety.

id say it with a singing telegram.
na na na NA ,
sit down honey , stop breaking s*it ..
im worn out , hubby iz havin a fit ..
that you need your independence , is unmistakable ..
weve found you a place , where everything is un breakable ..

you could make up your own lyrics i guess , but this might get you headed in the proper direction ..

Depending on her mental capacity, your relationship with her and many other factors I would imagine this would be very difficult. Would she understand explanations? Will she feel abandoned? Can she visit first, maybe an overnight?

I would talk to her about how grown up she is now and that brings very exciting things like being more independent and other exciting adventures. .. Such as living more independently in a new place.