A fughetta opens with a theme or a subject in any voice after which enters an answer (a theme in the key of the dominant) in another voice.
While the second voice plays the subject, the first one has a counter-subject - a contrasting melody with different rhythms than the subject. Counter-subject usually plays intervals of the thirds, sixths and suspensions with the subject.
The same happens during the last subject entrance. Here all three voices are sounding. The second voice has a counter-subject (the same or a new one). The trick with the third voice is to make it as stationary as possible.
The subjects of the fughettas can be taken from the chorale tunes (usually the first line), or they can be freely composed. The usual order of the subject entries is one of these four: 1-2-3, 2-1-3, 2-3-1, and 3-2-1.
After the last subject entry there is a closing cadence which wraps up the piece. Except of this cadence, there is a continuous flow without any full stop in all voices at the same time.
If you want to get more information about the fughettas and see how they are constructed, one of the best sources is the Weimarer Tabulatur by Johann Pachelbel, a master of fughettas.
There you will find many beautiful examples of this charming genre which you can imitate in your own improvisations or compositions.
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