The subtitution method is where you fill in a number into the place of the variable, the check the problem to see if the number fits by solving it..
x+2x=15
I don't see why someone would use the substitution method on this one. x+2x can be 3x because of how multiplication works like addition. Then you'd have 3x=15. You can then find the value of x by dividing each side by 3 so that the variable is alone. You'll have x=5
5x=2y+3
If the equations are part of the same problem, then you'll have 25=2y+3.
You could also solve this normaly, but I'll use the method you want this time.
You could put a number like 6 in for y. Then solve. You'll have that 25=15. Meaning 6 is too small, so then you could use a number such as 12. Solve, and you'll get 25=27. The answer would be too large, but barely. You could move down one, to 11. you'll have 25=25, so eleven is correct. Sorry if I typed too much.
If you wish to use the subtitution method, simply place a digit, that you think might fit, into the place where the variable is.