First the theory,
- The standard GLEE is a three- or four-voice a cappella song, although many examples also exist with from five to eight voices, and some early glees have basso continuo accompaniment. It is generally to be sung by solo voices. Glees often consist of several short movements.[1] The use of the countertenor voice, often on the upper part, is a particular characteristic of the form (the most famous exponent was William Knyvett), serving to distinguish it from German male voice music, in which the top part is taken by a tenor.- Thank you Wikipedia
Then the practice,
watch this: