The Ames guide is a classic comic’s tool. Also used for other forms of calligraphy, from drafting to decorative, it’s a simple variable ruling guide to render lines for lettering.
I’ve made a simple short video to watch here, and you can read the printed instructions most of them come with at the bottom of this page.
Dustin Harbin has posted a great photographic tutorial for using it too; The great letterer Todd Klein, who’s graphic I clipped for the thumbnails here – it’s lovely! – has a great instructional posted here; Another in cartoon form here on the Drawing Words & Writing Pictures site, from where I borrowed the hansom drawing of the Ames on the left! The book is the brain child of Jessica Abel & Matt Madden, I don’t know who drew the Ames though, that would one of them or someone on this page. All those sites are fantastic resources self directed learners should investigate!
A separate post about how to letter, and how I do so for my comics is coming.