Can you turn a normal mirror into a one way mirror?
You're likely going to run into some issues with the coatings that protect the metal layer. There are several types of glass mirrors:
https://en.wikipedia.org/wiki/Mirror#Types_of_glass_mirrors

Typically the layer that reflects the image is silver or aluminum. They are made by taking a sheet of glass, applying a layer of metal, then protecting that metal with additional layers of paint or protective materials. As such first the protective layer would need to be removed, it would depend on what the layer was made of to determine what would be used to remove it. After which some of the metal layer would need to be removed, probably by dissolving. What would dissolve it would depend on what it was made of. After this, a protective coating should be reapplied, or the lightest scratch will take off the mirror material.

It would be simpler to work with a fresh piece of glass and apply silver nitrate directly (as in photo making). Or even easier apply some two way film. Which is tint film that makes any glass a two way mirror.