Don't be alarmed if your dry pack roses come with the outer petals looking bruised and mashed.
These outer petals are referred to as "guard petals" and are left on deliberately to protect the inner petals during the shipping process.
Carefully remove the outer petals until you have a "perfect" rose.
The rose should feel firm to the touch. A "mushy" feeling rose is generally past it's prime and won't hold up as long in a design.
Once the guard petals are removed, you can cut the stem to the length needed with a sharp knife. Don't be tempted to use regular household scissors, because they can crush delicate flower cells in the stem and make it difficult for the flower to draw water up to the flower head.
White roses usually have a slight ivory cast to them and are not generally "paper white" like an artificial rose.