First remove at least the seconds and minute hands, the hour hand if desired.
Timex dials are usually held with tabs that bend over the movement plate. Lift the tabs as little as possible.
Most good watches will have two dial feet or posts with some sort of screw to retain them.
A common method on Swiss watches is a cam screw--this will have a very tall head, and a sharp ridge between the threads and the head. There is a flat removed from the ridge. These will be screwed in until almost snug, with the flat lined up with the dial hole. The dial is installed, then these screws are backed out--The ridge will bite into the dial foot, and backing the screw out will tighten the dial. To remove, turn clockwise until the flat aligns with the post.
A third, commonly found on Japanese watches is a cam near the edge. This will be pried away from the foot. (Picture will come later)