Hi
Not quite sure which bit is bothering you so excuse me if I state things you are happy with
ISA stands for International Standard Atmosphere
As such it is a purely arbitrary basis for setting out roughly what the real and variable atmos is like (as opposed to what the met man measures it as at any time).
Speaking from memory, at sea level the pressure in the ISA is defined as 1013.2 mb and the temperature as 15 deg C. If the met man measures the temp today at sea level and and it is 5 deg C then today is an ISA -10 day. If it was 25 deg C then it would be ISA + 10
So to use chart lines showing ISA + 10 and ISA - 10 amounts you first need to compare what the met man has measured/forecast the temp or where you are flying with the ISA value for the same height.
The lapse rate assumed in the ISA atmos is 1.98 deg C per 1000 ft up to 36000ft and zero after that (I think but I am too lazy to go and check).
But I am sure you have a table giving all the ISA values of pressure and temperature against height. If not that is where you must start ie get yourself one from just about any book dealing with flying.
Finally if you are happy that (say) your actual conditions at FL300 or wherever are ISA+7, then you look on the chart between ISA and ISA + 10 and estimate 70% of the way from the ISA line towards the ISA + 10 line and read off the value of whatever it is your chart is for.