Since harmonization is the foundation of historical preluding, the basic triads will be the first elements to learn.
Major and minor triads can be practiced up and down the keyboard as shown in the example below. It is recommended to learn the corresponding chord symbols and to become familiar with the differences between major and minor triads.
The following example shows two versions of a triad spread out over the entire keyboard.
If the hands easily span an octave range, this version could be more suitable:
Each triad has a root postion and two inversions, which can be practiced as well:
The number of chords to be learned varies according to one's learning speed. But as soon as one is familiar with a number of chords and their symbols, small experiments can be performed in a playful way with limitless possibilities. For example, chords can be alternated while crossing the hands, and various parameters such as dynamics can be used to create short improvisations.
The improvisations can be performed within or out of the context of 18th- or 19th-century music; it's mainly about exploring the chords in a personal way, so one could also use a more contemporary approach.
The following document gives a full overview of all triads and their corresponding symbols:
August Friedrich Christoph Kollmann's (1756-1829) An Introduction to the Art of Preluding and Extemporizing (1792) shows several examples of creating preludes from a single triad.