Achim's guide is good. In traditional Palmer and Spencerian, ascenders are 3 times as tall as the x height. So if your letter "a" is 2mm tall, then the letter "h" would be 6mm tall. The way that calligraphy program expects you to input the lines for ascenders and descenders is confusing, so let me spell it out to be clear:
Spacing between verticals -- I like 10 nib widths here, but anything should work.
Angle -- 38 degrees
Nib Width -- 1mm
Ascender Height -- 0 (This is the amount *above* a capital letter. But caps and ascenders are the same height in Palmer).
Capital Line -- 4
x-height -- 2
Descender Height - 4
White space -- 4
You could change x-height to 3 and caps and descenders to 6 if you wanted to write larger. Personally I think that 4mm x-height is much too large for Palmer, 2mm seems about right for writing, and 3mm might be good for practice.
I posted files for both the 3mm
As far as how to use the guides, I think you are right that a picture is very helpful. I don't have one at hand, so please bear with my description. Each letter without ascenders and descenders fits within the x-height lines. A letter like "h" will go up to the very top line, and a letter like "g" will descend to the bottom line. The downstrokes
of the h and g should line up to be either on or parallel to the angle lines of 38 degrees.