When you are substituting equations, all you are doing is plugging in variables. Lets do an example..something random but easy...
y = 2x - 3
4x + 2y = 10
So since 2x + 3 equals y we are going to plug in those numbers into the second equation.
4x + 2(2x - 3) = 10
Now we distribute
4x + 4x - 6 = 10
Combine like terms.
8x - 6 = 10
Add 6 to both sides.
8x = 16
Divide.
x = 2
And thats how you substitute equations.
I hope this helps love! :)