Place gravel. Scoop out letters in the gravel. Pour liquid cement in the grooves and over that sprinkle coloured sand. Let it dry atleast for a day. Then gently sweep off.Whatever loose sand that is scattered will be swept away. The sand stuck to the cement will highlight the letters. Good Luck.
Big rusty metal letters would look great. Maybe there is a metal works company that might do it for you. My son works for a metal works company and they have done some specific things for us. I don't know whether it's a matter of "who you know" but maybe it's worth a try. Just as thought!! Good luck with your project.
Photos of the site would have helped, but on any significant slope, loose gravel would need to be held in place somehow. Trenches, various types of frames (wood, stone, metal, etc.) or binders such as concrete or mortar could all keep the pebbles in place, but all these methods would work well without pebbles at all! The sign can lay flat against the surface of the slope, or it can be raised and propped up to a higher angle. For example, the upright, vertical letters of the famous 'Hollywood' sign.
You never mentioned how permanent or temporary, these letters would be, but if you intend them to remain in place indefinitely, sturdy structural materials will be required.
Of course, there many more options for temporary signage, involving painted tarps, poster boards, and the like.