I will focus this answer on ground planting (as opposed to sowing in cell pots, or planters)
Plant them in the spring after frost has passed, but the ground is still cool. To sow in the ground, work up the top 3-4" of good topsoil and rake it smooth. You want a flat, even seedbed. For a row, create a furrow >1" deep where the row is to be, and place seeds in groups of three, every 12-16" in the row (this will ensure you will have at least one decent plant per spot after thinning) cover with 1/2" of soil and lightly tamp and water.
To broadcast, sprinkle the seeds at about 40-50 seeds per square yard, in a consistent layer, and rake lightly. Water the entire area. You should have 90% germination after 3 weeks. These are somewhat erratic, and some seeds will sprout before others.
It's important that you know what a Cleome seedling looks like, so that you can identify them and avoid disturbing them while performing weed control (here's a picture showing the cotyledons (first to emerge) and early leaves):