It's especially helpful that you're a family with DC, as it will be beneficial if the rescue can have a good understanding of how the dog interacts with DC.
The rescue should have a foster co-ordinator who should be your guide/liaison. I'd expect them to give you as much info as possible on the dog's background (if this is known) and any info on existing preferences for food, medical/vax status etc.
If neutering/chipping is being done while with you (often preferable as better postop care than in kennels) they should advise on which vet practice, and should also clarify the protocol on if dog is ill/injured e.g. do they require you to take to specific practice or is your own existing one acceptable? This is one of those issues that if you foster regularly can be a real PITA if you are having to go a long way out of the way just for one dog and can't use your own practice.
Dogs are at the greatest risk of escape/loss in the immediate first hours after rehoming whether with foster or adopter so make sure all family members are absolutely drilled on protocols for door opening, and that dog is wearing ID. Collar plus harness with double lead may be a good idea for walks initially until you have better idea of your relationship. It's also worth using the FB group for Secure Fields to see if there is a suitable secure field you can use locally for off lead zoomies, and to practice things like recall.
It can be helpful to teach recall using a whistle as this is not then linked to either your voice or to a specific name, which then keeps options open when dog is adopted.
Once he/she is settled in, take lots of pics. These are useful for the rescue to share on social media and help build interest in the dog. Although you will be sticking with your own dog's routines it's great to try to introduce a variety of different situations to see how your dog copes.
Typically at this time of year, make a note of how they react to fireworks/loud bangs, and see how they get on with travelling in the car, meeting other dogs, meeting people, (both at home and away from home) behaviour at the vet's, maybe things like a small local dog show to see how they manage. Are they comfortable with grooming/washing? Depending on breed, this could be especially useful.
You will also need to report on diet/food preferences etc. Basically imagine what the adopter would need/like to know to give the dog a happy home and good start.
You are doing a fantastic thing - well done. It's hugely rewarding and we ar very lucky in having made good friends with people who have both fostered our dogs before they came to us, and who have adopted dogs we have fostered. Good luck!