If it is still frozen it will probably be fine. I have found that if I soak them with the hide off and it is thawed, you tend to get more oils to stain seems in the skull.
I'm probably much more particular than most are for their deer skulls, so I spend quite a bit of time on mine or those for friends and family. I try to get the hide off and as much flesh as possible taken off the skull as quickly as possible. I macerate the skull in a big turkey pot on low heat for a couple of hours. If you have a pressure washer and especially a heated pressure washer, that typically works best. Like Bill had mentioned, be really careful with the pressure washer if you cannot control the pressure. Most of the time I just clean it manually with some picking tools and a garden hose. I like to change out the water several times when it is macerating, adding dawn dish soap each time. I do not add borax or anything else to the pot when it is cooking.
I use Salon Care Volume 40 Creme instead of mixing a paste. It is about the same cost and a little goes a long way. I've done about 15 skulls and still have half a bottle. One trick I've used is to use a full-spectrum grow light for the bleaching. You can use the sun, but when I'm doing the skulls in the fall/winter, there often isn't any and I like to bleach the skull as quickly as possible.
I have buried a few skulls and that is by far the easiest. I'm typically too impatient to wait for the spring to have the skull back, though. Also, I have had a few not bleach as well so I stopped doing that.