"Dead simple.." vs. "Really simple.."

Dead simple is very informal. Whether or not you use it in preference to really simple depends entirely on the impression you want to make on your readers.

The "Dead" adds strong emphasis, and any reader who does not find it simple will think you arrogant, assumptive, or both. I'd avoid it. Just plain "Simple" would be my choice.

What you find simple others may not. You are walking a line between encouraging your audience to continue without fear of complexity, and not sounding like what may rocket science to them is merely falling off a log to you.