Read the bottle, most say 1 bottle for twenty gallons.
If you have a 20 gallon tank, then use 1 bottle.
If you have a 10 gallon tank use 1 / 2 bottle.
It does not matter if you put the injector cleaner in first, or the gas in first.
Good quality gasolines from distributers such as Shell, Chevron, Unocal, BP, Amocco and others contain detergents that clean your fuel system.
Budget priced or thrifty gasolines may not have these same detergents.
If you want to save money, but can't afford the higher quality gasolines all of the time ?
Do the 5+1, five tanks of thrifty and then 1 tank of the good stuff, this way you will be cleaning your fuel system at regular intervals and it won't break the bank.
I agree with the first answer, no oil in the gas tank.