Measure the length and height of the wall you need to cover, using a tape measure.
Obtain fabric or curtains that can be adjusted to that length or are a little larger. Curtain panels work well in this situation because you can purchase as many or as few as needed.
Screw a curtain rod into the wall above the mirrors. If the mirrors go to the ceiling, you can use a piece of fishing line strung across the wall and anchored at the walls, perpendicular to the ones you wish to cover. This can be done by tying the fishing line into a knot on hooks placed on both perpendicular walls. Use a turnbuckle attached to the line, if needed to tighten the line.
Hang your curtain panels or fabric from the rod or line using hooks or the grommets already in the panels.
Arrange the panels or fabric so that they cover the mirror.
Clean the mirrors well with a glass cleaner or white vinegar.
Wipe them completely dry before painting.
Prime the mirrors with high-quality primer to give the paint something more substantial than the glass to adhere to. This will also give you a thicker finish that does not show through to the mirror.
Paint over the mirrored wall with latex interior paint, using a brush or paint sprayer. A roller will leave a texture on the mirrors, which you may not like.
Add interest and depth to the wall by then adding pictures or other collage items.