It's Very easy. First use a grinder with a cut off wheel. Make a square using a strait edge with a black marker. Grind down about 1/2 to 3/4 inch. Remove marcite with a hammer and chisel down to the concrete. Clean area with a wire brush and wash with water and allow to dry.
Marcite patch comes in 80lb pre-mixed bags. Call pool companies around your area. They can order it for you.
You will also need a bonding agent such as weld crete. That only comes in 1 gallon cans. you can get this at hardware stores.
Paint on the bonding agent and allow to dry. Mix up enough pre-mix marcite. This stuff sets up in about 3 to 5 minutes so work quickly. Use a sponge or trowel to smooth the surface. That's it.
For cracks. Grind the crack 1/2 to 3/4 inches and repeat the above steps.