Do not put towels or anything pups might get tangled in or under, as that could cause you to have a pup get suffocated/sat on. Towels are just for rubbing the pups dry. Wet wipes, non perfumed are best for Mum. A sponging down with water with non rinse baby bath liquid if she gets very messy.
The whelping box should have just paper (I use shredded as it is so easy to remove soiled easily and the wet passes though more). During delivery I will have a heat pad in there as my bitches would not tolerate having their pups removed between births,a nd this helps keep them warm and dry.
After whelping heat pad in the middle under a piece of Vet Bed, which as has been said should be washed daily and initially should be white or a light colour so that you can monitor colour of discharge.
If it is very warm and the bitch is digging her bedding up it may be safer to remove the vet bed, but I have usually found they stop digging in a couple of days,a nd pups get around better on it and it helps keep them warm while the bitch is away from them (though they will gravitate to the heat pad if it's cooler). If I have to remove the vet bed in the first few days I put a cover on the heat pad (pillowcase).
I have quite a few white pieces, but this year I used the non slip backed in yellow and the pups looked nice on it for photos.
If it gets very warm beware of the pups getting overheated (then you may need to turn off heat pad, or even use some method to cool. A thermometer taped to the inside of the box in the time before their eyes open may be a good idea.
I have had two July litters and at various times they needed cooling. I froze water in the square 1 litre squash bottles, wrapped in a towel and had them at the sides of the whelping box. A fan can be used close to but not directly onto the puppies.