I have never made any garments to even think about how I would deal with the collar. When I watched videos on the internet the lady put the interfacing on the t-shirts before she assembled the quilt - so that is why I put the interfacing on before I trim the pieces and sew them together. I am making a memory quilt from my nephews tshirts.... I have quite a few. I wanted to wash the tshirts after interfacing before I go any farther in case the interfacing doesn't last through the wash. Then I would know that I needed to redo the interfacing before I trimmed and proceeded to sew.