Spread the concrete around with a shovel and press it against the sides. Then mix another batch. When you have about half the form filled, use a 2 x 4 screed board to level the surface, but leave it slightly high. Fill the other half of the form in the same way. A larger slab requires rebar or steel mesh to help control cracking, but a small slab can do without that kind of reinforcement.
Then, steadily saw the screed board side to side while moving it along the form. If you find a low spot, fill it and saw through that area again.