So,

I bought an EOS3 in almost new condition. In the near future I plan on buying the 50mm f/1.8 EF lens, which I have used in the past with an Elan 7e to great effect and with results that outperform my own abilities as a photographer. It's more than good enough.

But, I read somewhere that it's possible to use M42 screwmount lenses, like a Pentax M42 50mm f/1.4 lens via an adapter. What do I need to make this work?

Thanks for your help. I'm used to Pentax where everything is compatible with everything. The Canon world is a little confusing to me.