Fever thermometers can be used under the armpit, in the mouth, in the ear, on the forehead or rectally. The most accurate and shortest measurements are taken at the last three places mentioned. Basically, you can put the thermometer under your armpit.
This is a popular spot, but it also gives the least accurate maximum reading and takes the longest, about 8 minutes. However, the thermometer in the mouth, i.e. orally, is even less accurate and can take up to 5 minutes. The best and quickest method is therefore to use a clinical thermometer for the bottom.
Ear thermometers and non-contact forehead thermometers are also good thermometers for taking a quick temperature.
For babies and toddlers, it is best to use the rectal method, as this is the most reliable.