Next we add the straps and fasteners, I chose buttons since this particular pair of pants had lots of spare buttons that I could salvage. But you are free to use any sort of fastener (clasps, clips or buckles) as you wish, just try to avoid velcro (as suggested by criggie since they get filled with fur after a month or so)
These 3 locations should be enough to help keep the vest in place.
At this point you may want to try it on your dog to see how long you need to make the straps. Sew the straps to the main section and add the buttons and buttonholes as needed.