You buy rooting gel or powder. Some people say gel is better. All your utensils need to be clean, as in boiled or disinfected, the cut can easily mould.
You snip the plant beneath the second (or third or whatever) branch. You move it over to a bowl of water, re-snip the cut under water as slanted as possible to make a large open wound, from which the roots will come from. You do this under water to avoid air bubbles in the stem. You let it sit in water for 10 mins. Then you dip it in the gel or powder, and carefully put into rockwool. You can use weak soil, but people prefer to use a medium that has no nutrients at all and is PH neutral.
Clones do not want nutrients.

You put them in a seedling tent, and spray the insides with water. The air in the box must be kept moist. You try to not spray directly on the plants as the mould easily.

When they start to root you carefully transplant them to your intended medium, being very careful about no nutes, and careful with the light strength until they start to show signs of beginning to grow.

You should read a guide, it can be a little tricky.