You will need to use sulphuric acid. Hcl will not do as amphetamine hcl is hygroscopic, it will just be a sticky mess.
When measuring with your ph strips you will be aiming for a reading of 7 or slightly below.
I think you should study a little organic chemistry before you attempt this. Specifically ph; acids and bases. If you don't understand the chemistry you are liable to misinterpret the instructions no matter how concises, and fuck up.