Rusty , mark your cut line on the canvas , then brush over the line with 50/50 water (or even thinner) thinned White elmers glue … ( non waterproof) let that dry , then make your cut on the treated fabric line …. No threads

… then when you glue the canvas down the white glue just blends in with the other glue … clean seams … OK ?
Good ( Pretty) covering is an unhurried job …. take your time , neatness counts ...