Bring the liquid to a boil (broth, stock, milk, or cream).
Whisk the eggs separately in another bowl. Using eggs whites or egg yolks will yield different results, but most tempered eggs use the entire egg.
Slowly add the hot liquid to the eggs a little bit at a time and whisk constantly. The key here is to slowly bring up the temperature of the eggs without cooking or scrambling them.
Notes
Keep whisking until the mixture has reached its desired thickness.