Well basically you are plugging in variables. Lets use a random but simple system of equations.
y = 2x - 3
6x + 8y = 20
Now you need to plug in the top equation, which equals y, into the bottom equation.
6x + 8(2x - 3) = 20
6x + 16x - 24 = 20
Combine like terms.
22x - 24 = 20
Add 24 to both sides.
22x = 44
Divide.
x = 2
Now plug in 2 in any of the original equations.
y = 2x - 3
y = 2(2) - 3
y = 4 - 3
y = 1
So your solution is (2,1). And thats how you use substitution.
I hope this helps love! :)