It is best to schedule insertion for when the woman is tehorah, ideally shortly after mikveh immersion. Blood from the trauma caused by insertion does not make a woman niddah. Please see our artice on IUD Insertion and Removal for a detailed discussion of these halachot.
Following insertion, by relying on the leniencies of stains, a woman can often avoid becoming niddah due to the staining from the Mirena.
Please note that some staining associated with Mirena can make a woman niddah, and can sometimes continue for months! To avoid unnecessary questions, please review the laws of stains, as by reading our site’s articles “Stains” and “Toilet Paper“.
Please write back with any further questions.