I assume it's a rectangular bench with two footed legs near each end. Also, you probably want the legs to have a curved shape.
I would make the legs first using styrafoam "blue board" which is 3/4 in thick and easily cut with a jig saw. Draw up a pattern in the shape you like. Cut out the leg shape leaving a flat female mold. If satisfied with the shape, make several more indentical pieces tape them (at least three pieces) together to give the desired leg thickness.
Lay them on a flat surface covered with a piece of dry cleaner bag so the mix will not stick. Put strengthening fibers in the mix, and also use wire cloth or chicken wire to reinforce the piece. If satisfied, make another leg.
Make a rectangular frame the thickness desired for the top. With the frame on a piece of plywood fasten two pieces of wood a little larger than the thickness of the leg to the board with screws so these pieces can be removed shortly after molding. Again use fibers and wire for strength and cover with dry cleaner bag material. fill the mold with mix. Tamp it into all corners. After it has set up for a couple of hours, carefully remove the wood strips. This will leave two cavities about the size of the top end of the legs. Insert the two legs into these cavities and carefully work some of the mix in and around the legs in the cavities. You must be careful to keep the legs vertical and square to the top while curing. You may also want to lay two pieces of wood length wise on the underside of the top between the legs fill with mix. This will give a rib connecting the legs adding rigidity.
After the upside down bench cures for two days or more, carefully remove the wood frame and turn it over. The areas where the legs stick out of the top can be filled in with mix, smoothed off and finished along with the whole top.
That is only one idea. Someone may very well have a better one. Always think ahead about keeping things square and be sure the mold can be easily removed. good luck

The above instruction was for a hypertufa bench not concrete.