Which Martin Lake? There's four in Texas. Are you talking about the one near Tatum?

Really, no matter which one it is, you'll need to fish in really shallow water, preferably where the main feeder river or creek is. With the warm weather this week, the shallows are going to be on fire. I would search for an area with a muddy bottom instead of a rocky bottom. The mud absorbs heat better and the shad will be basking in those areas. You don't really need to chum the area unless you're preferring to catch channel cats, in which case you could. Try to get some shad, even if it's frozen. The fish will really be going after them. Good luck!