Contact an expert such as a structural engineering to help determine the gradient of the garage. This is essential to establish whether the garage will be flooded once the concrete is poured. A gentle gradient with a slope towards the garage door is most preferred as this greatly reduces the chances of your garage flooding thus protecting the items stored in it.
Dampen the wheelbarrow prior to pouring the sand, gravel and cement to form concrete. Pour the gravel, the sand and the cement in the ratio 3:2:1. This implies using 6 spades of gravel, 4 of sand and 2 of cement then mixing them up. Add water to the mixture using the bucket and continue mixing using the shovel. Continue adding water till the mixture forms a fine paste.
Stretch a string across the garage to give the intended height of the concrete. Pour the concrete on the surface of the garage furthest from the door. Smooth the layer of the concrete to the height of the string using a trowel. Continue mixing the concrete, pouring it and smoothing its surface until you cover the whole garage surface. In case the concrete in the wheelbarrow seems to be drying, add some water and mix it using the spade to make it a fine paste again.
Spray water on the concrete floor every morning and evening for six successive days. This ensures the concrete dries slowly thus becoming strong.
Clean the garage floor of any dust which may have accumulated once the six days are over to commence the process of painting. Use the brush to apply an even layer of paint again beginning from the furthest end from the garage door. Upon completion, allow sufficient time for the floor paint to completely dry (not less than 24 hours).