First you must have an erection.
Open the condom and figure out which way it unrolls toward the outside.
Pinch the tip of the condom with two fingers (to make a reservoir for the sperm) and place it on the tip of the penis.
With other hand, unroll the condom over the whole penis.
After the end of the intercourse, you must remove the penis from vagina while the condom still on the penis. Only then, it will be safe to remove the condom.