Your phrasing is not correct. You are really asking how to write chemical formulas, not make them.
Here's a website to help:
A chemical formula is a symbolic representation of chemical reaction. This representation is in the form of an equation. The left hand side of the equation represents the input substances for the reaction, followed by an arrow pointing to the right, and then the resultant output of the chemical reaction. For example, chemical formula for burning of hydrogen, is as follows:
2H2 + O2 --> H2O
This formula indicates that two molecule of hydrogen, each containing two atoms (H2) combines with two molecules of oxygen containing two atoms (2O2) react to give two molecules of water, each of which contains two atoms of hydrogen and one atom of oxygen (H2O).
The main difficulty in writing the chemical formula is to balance the left hand and right hand side of the equation. This means ensuring that total number of atoms of each element present during the reaction is same on both side of the formula. This is called balancing the chemical formula. This requires that number of molecules of each substance involved in the reaction is multiplies by appropriate whole numbers that result in balanced formula.
It is possible to develop some kind of mathematical algorithm to balance the chemical formulas. bur it can be doe more easily by method of trial and error. For example for the above formula we can start with a formula one molecule of every substance. That is:
H2 + O2 --> H2O
In this formula the number of atoms of hydrogen are balanced, but number of atoms of oxygen on left hand side are twice that on right hand side. So we double the atom of H2O on the right hand side.
H2 + O2 --> 2H2O
Now atoms of oxygen are balanced but atoms of hydrogen on right hand side are twice that on left hand side. So we double the molecule of hydrogen on left hand side. This gives us:
2H2 + O2 --> 2H2O
This balances atoms of hydrogen as well as oxygen. Therefore it is the correct formula.