First of all, to do letters, you need to create a pair of rulers to guide you. If you're going to write in a slanted or cursive hand, add some slanted guides across the two rulers as well, for example:
Then zoom in so that the height between the rulers corresponds to your most natural hand movement range, adjust width and angle, and off you go!
Probably the first thing you would do as a beginner calligrapher is practice the basic elements of letters — vertical and horizontal stems, round strokes, slanted stems. Here are some letter elements for the Uncial hand:
If your hand is comfortable on the tablet, don't move it. Instead, scroll the canvas (Ctrl+arrow keys) with your left hand after finishing each letter.
If your last stroke is bad, just undo it (Ctrl+Z). However, if its shape is good but the position or size are slightly off, it's better to switch to Selector temporarily (Space) and nudge/scale/rotate it as needed, then press Space again to return to Calligraphy tool.
Having done a word, switch to Selector again to adjust stem uniformity and letterspacing. Don't overdo this, however; good calligraphy must retain somewhat irregular handwritten look. Resist the temptation to copy over letters and letter elements; each stroke must be original.
And here are some complete lettering examples:
Calligraphy is not only fun; it's a deeply spiritual art that may transform your outlook on everything you do and see. Inkscape's calligraphy tool can only serve as a modest introduction. And yet it is very nice to play with and may be useful in real design.