There are a variety of techniques for seating veneers. Some people like to do 6-10 all at once, while others seat them two at a time. I am in the group that likes to seat restorations a small number at a time. One of the challenges when doing this is to not get resin cement on the adjacent preps. This resin can prevent the restorations on those teeth from seating or it will have to be mechanically removed. The use of a matrix between is an option for keeping the resin from contaminating the adjacent prep. Both metal and Mylar matrices are difficult to keep in place and pose their own host of challenges as we try and etch, place dentin adhesive and seat the veneers.
Years ago I learned the trick of using Teflon plumbers tape as a matrix when bonding in restorations. The Teflon tape is designed to help create a seal when two pipes are screwed together in plumbing. If you have ever replaced a shower head you have wrapped plumbers tape around the threads on the end of the pipe prior to screwing on the new shower head. Dry the adjacent preps or teeth that you want to protect. Take a piece of plumbers tape that is 1/2 to 1 inch long. Place it in the interproximal embrasure and pull it over the teeth or preps with the ends away from where you will be working. It will resemble placing plastic wrap to seal a bowl of food. The Teflon will stick to the teeth and stay in place. You can now etch, rinse, and seat your restoration without worry of the resin adhering to the adjacent teeth and without having to hold a matrix in place. When you are done simply pull the Teflon tape off.