it's actually pretty simple, just remember to pair the correct resistance to the correct voltage. general rule of thumb is as follows.
lower ohm carto/atty=lower voltage
higher ohm carto/atty=higher voltage
for instance, a "low resistance" atty/carto (1.5-2.0ohm's) you would want to use basically at 4v or less, this will give you a very warm vapor.
for a cooler vape try a 2.5-3.0ohm atty carto at that same 4v or less
at 3+ohm's 5v or higher will give you a warmer vape.
but just remember that basic rule of thumb, if the resistance of the atty/carto is low you want to pair it to a lower voltage. if it's high pair it with a higher voltage. most sites if they don't list the exact resistance of the atty/carto will at least tell you it's either a lr-low resistance (1.5-2.0ohms), sr-standard resistance (2.5-3.0ohms), or hv-high voltage (not sure y some sites call it that as it should be high resistance not voltage but anyways 3.5-4.0ohms)