where should the strap for tennis elbow be placed
People can go to any CVS or Target and get a nice tennis elbow strap these days. Counter-force tennis elbow straps are effective at allowing patients with lateral epicondylitis to produce more grip strength with less pain. The strap should be placed around 2.5 cm distal to the lateral epicondyle. While it is unclear if the size of the strap is important, the study used straps that were between 5-8cm in width. Considering there is some conflicting results in the literature, I would recommend you try to replicate the width of the strap.
A counter-force tennis elbow strap can be thought of as your fingers on the neck of the guitar (your forearm) and the extensor muscles, especially the extensor carpi radialis brevis, would be the guitar string, thus reducing tension of the muscles as they attach to the lateral epicondyle.