How to Make concrete slab under a gate?

by LG
Have a gate that I need a rectangle slab under it to prevent animals from digging holes to come in yard. Would like steps to complete--have never done such? What type of forms do I build? How do I mix concrete to mix & pour? Would like rectangle to be approx. 14-15" X 40"--can adjust if needed to go bigger or smaller? Any help would be great.

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