I regularly do a variation of a hammer donning ritual I read a long time ago in Thorsson's Book of Troth. Basically hold the mjölnir in front of you and say something along the lines of "may this day bring the best of luck, greatest of deeds, new adventure, more knowledge, and the blessings of Thor/Gods (other names you wish)"
When I first get one I hold it in my hand and pour out a libation to Thor and ask him to bless and protect my self, family, and friends through the mjölnir and through my life. That his strength and honor hallows the mjölnir and raises my strength and honor. I use it to bless house/land regularly after that and for any other smaller and more common ritual I do in place of a full-size hammer.
I don't think I've ever done it the same way twice
Something along those lines, moved and altered as inspiration takes you, would be good in my opinion. You could ask that your mjölnir is charged with strength, honor, abundance, goodness, etc. instead of "blessed" with them.... For example. What means most and flows through you personally.
I always have an experience of a sort each time.
a feeling of power and peace usually.