First, find high and low areas using a leveler. Mix compound with water and pour where needed; the self-leveling compound will fill in lows by itself. You might have to use a trowel to 'push' the compound if needed.
When laying self leveling compound, I prefer to spread it using my hand and disposable rubber or plastic gloves. This allows me to feel for flaws in the floor, and correct any issues quickly. After application, I go over once with a mason's trowel for a final smooth finish.