Help with True Bypass Loop Circuit w/ Mod.

[NHDAVE]

Okay I can't buy what I want so I'm hoping it will be possible to make it.  I have studied and understand the True bypass loop selector.  What I want to do is add a Blend knob for the effects mix in line.  What pot should I use and what else do I need to consider.  Bottom line ..I've done a few mods on pedal, but would this be out of my league for a first time build?........Any input would be tremendusly appreciated.  Thanks.

[sodapop]

So you want to parallel the effects wet signal with the guitars dry signal and sum them back together in mono?