Guptill's book is really a great way to get started! (understandably a bit dry but really solid training examples to warm up with!) Peter Han's Dynamic Bible, and Draw a Box are also great ink resources, most of the training is with Micron so you build line confidence quickly. My advice is to start to get as comfortable as you can handling the tools, they're surprisingly forgiving, and really dynamic. Jump in and start making lines. Fill pages and pages with experiments, thick and thin lines, try hatching, draw spheres, try rendering them, etc just start drawing with them. Try controlling the line weight or bounce of the line. Find out how long a line you can pull before the nib runs dry and needs a dip, etc. Pushing the materials to the limits will help you figure out the limitations of each nib, and brush, and you'll start to build the dexterity required to handle them. Drawing or sketching with Microns and Brush Pens is a great way to start building confidence with ink, and you don't have to worry about knocking over an ink well! (Pro Tip: Mark Morales literally tapes his ink well down to the desk when he works!) so just start putting in some mileage with the quills, and brushes to see what they're capable of, and have fun! Here are some free inking episodes from David Finch's Creating a Comic Course, https://www.proko.com/course-lesson/traditional-inking-tools-techniques and Marvel's The Art of Storytelling with Mark Morales https://www.proko.com/course-lesson/bring-your-pencils-to-life-with-ink-comic-book-inking-part-1 https://www.proko.com/course-lesson/learning-from-inking-pros There are a few Watt's Atelier videos where Jeff inks worth checking out as well for technique and overall inking philosophy https://youtube.com/playlist?list=PLM_bjhD1WV0QA3fPjTUhzoi-EHlK_YR6L&si=LdjRh3j0MRN9ZICj

I've been doing a lot of pen & ink lately. The Guptill book is really useful if you actually do all the exercises hands-on! and imitate sections of the art. Try doing some drawing basics exercises in pen, e.g. shade spheres and cylinders, create a 10-grade value scale with various line widths and hatching, etc.. Also, do ink master studies! I've done Dürer, Carracci and Ostade studies so far. You can do a master study of just a small section of a larger work to understand the techniques. I'll share a couple pages of some of my exercises based on Guptill...

There is a new course on Proko by Len Nicholas called Line Weight Fundamentals that may be worth checking out. The course is based around a three line principle of thick, medium and thin line weight and how and where to use them.