Well I'm not really sure if they do this for A level chemistry. I don't remember seeing this back then, it's from my 1st year in university.
Acidity is caused by hydrogen ions (protons) in the solution. The pH is given by -log (H+) where H+ is the concentration of hydrogen ions in the solution. So the pH is essentially a measure of the concentration of hydrogen ions in a solution. So to make the solution to the required pH you would need to work out the concentration of hydrogen ions you needed initially. Concentration = number of moles/volume. So rearrange pH=-log(number of H+ions/volume of solvent) to -volume*10pH=number of H+ions I think.
Strong acids completely dissociate so if you used a strong acid you would be able to gurantee that all of the hydrogen atoms in the molecules dissociated to give hydrogen ions. For example HCl contains 1 hydrogen atom, therefore 1 mole of HCl would dissociate giving you 1 mole of hydrogen ions.
So once you know the concentration of hydrogen ions you need from the initial part make up a solution of that concentrion of acid.
These are random made up values, but you find for pH X you need a concentration of hydrogen ions of 3 moles per litre. You are using Hydrochloric acid to make the required concentration which you know completely dissociates because it's a strong acid. So you know 3 moles of HCl will give you 3 moles of hydrogen ions in solution. So you dissolve 3 moles of HCl in the required solvent.
Or for another example, you are using Sulphuric acid which you know completely dissociates but has the formula H2SO4. As it has 2 hydrogens which completely dissociate you know 1 mole of H2SO4 gives 2 moles of hydrogen ions so to make your solution of pH X instead of using 3 moles as you used for the HCl you would use 1.5 moles. (Of course remembering never to dilute concentrated Sulphuric acid solutions with water.)
I hope this helps, though personally I think this might be a bit advanced.