As this is a "cheap and easy" way to do it, there are some drawbacks;
There will be no autofocus, and the DoF (Depth of field) is very very shallow.
Step 1: What you need.
I am using a lens from a Canon 1000f, I picked this up for about £10 (camera included)
and this lens is ideal as there is no need to unscrew anything.
And any scalple, x-acto knife, craft knife blade will do.