You should be able to prepare the white sheets before the nine days; therefore, we do not see the need to do this during the nine days.

In general, one does not put fresh linen on the bed during the nine days. However, it is permissible to change to clean sheets when halachically necessary, when beginning the seven clean days or to avoid problems with stains. Thus, changing to colored sheets is permitted after immersion. (If you plan ahead, you can avoid using clean colored sheets. When you remove the colored sheets from your bed before the seven clean days, you can put them aside unlaundered, and re-use them after mikveh without laundering.)

If your husband also has white sheets, then you may change both beds, unless you never sit or sleep on his bed. But there is no halachic requirement for him to use white sheets while you are counting seven clean days.

You can do your usual preparation before using the mikveh as far as bathing. There is a debate regarding shaving legs, etc. Therefore, you should contact your rabbi as to how to proceed.