Thermistors are calibrated by placing them alongside pre-calibrated temperature sensors in a controlled environment (an enclosed, insulated, fluid filled-tank or gas-filled chamber is best), then adjusting the temperature and noting the thermistor’s response over a range of different temperatures. You’ll only need to perform the tests within the expected range of temperatures the thermistor will be used for, with perhaps a small amount above and below. Knowing the range of operating temperatures will determine the exact type of thermistor you buy.
It’s important to recognise that thermistors do not follow a linear path, so knowing just two temperature/resistance pairings will not be sufficient to interpolate the rest of the values over a range of temperatures. The relationship is a curved form, which means you’ll need at least five (but the more the better) points to plot the curve on your thermistor graph and predict the points in between. Also, there will be a range of temperatures where the temperature/resistance curve will be steeper, and therefore less precise over small temperature changes. If you’re calibrating a thermistor for a very temperature-sensitive application, make sure you choose one that’s most accurate within that range.