Hmm. The OP asked a confusing question.
how a front mounted shutter is connected to a barrel lens?
Shutter in front of lens? The shutter is attached to a ring that slips on the front of a lens. See section 8 of
https://galerie-photo.com/baby-bertha-6x9-en.html for an example with discussion and pictures.
Lens in front of shutter? The lens goes in an adapter that screws into the front of the shutter. Look in
http://www.galerie-photo.com/telechargement/dan-fromm-6x9-lenses-v2-2011-03-29.pdf for examples and pictures. There are many ways to do it.
(working with someone on a DIY focal plane shutter)
Really? The typical lens sits well forward of the focal plane. An FPS sits very close to, um, er, ah, the focal plane. That's where the film is supposed to be. OP, did you mean a roller blind shutter, also called curtain shutter, or a guillotine shutter or a rotary sector shutter?