I usually start with pencil sketches to get some rough ideas and quick forms down on paper. This allows me to see if my idea may work and how some of the letters might relate to each other with design characteristics and proportions.
If it’s a hand drawn font, then I’ll typically draw all the base characters on paper with pencil and pen, and then scan and trace in Illustrator, eventually pasting into the Glyphs font app. If it’s a precise, multi-weight family, I’ll still do the pencil sketching but usually then go straight into Glyphs to draw the vector characters, using the sketches as a reference as I go along.
After completing all of the base glyphs and components, I move on to spacing and kerning and any other OpenType features. Along the way, there is exporting and testing of the fonts in progress to make sure they’re working properly and look as desired.