This kind of hard to explain without an example, so I'll do my best...
All you have to do is isolate a variable on one side of the equals sign without a coefficient. Ex: x=2y+3 The x is all alone and ready to go into the second equation.
Substitute into the second equation. Ex: 2x+5=3 Substitute: 2(2y+3)+5=3
I hope this helps!
