The Chortle is really a subset of Expressions of Mirth. The technique has defining qualities, though there is plenty of room for individual interpretation.
The Chortle should be, but is not limited to: Sniggering, but in a congenial tone, and/or a Giggle-y rhythm, slightly choked off, and/or volume control, not to be confused with a Guffaw. It sounds easy, but it does take practice.
Ultimately, the Chortle is motivated by spontaneous Merriment, but vaguely discreet. Keep at it; you’ll master it with time.