Hello Pancho- We are probably a similar vintage, and I am also a left handed writer. I have recently been training myself to change from side writing to under writing, and am enjoying writing now for the first time in my life, especially with fountain pens.
I have purchased a number of guide books that have been helpful in different ways, and these are the ones I would suggest-
Write Now, by Barbara Getty and Inga Dubay is very helpful and addresses the particular issues facing lefties. Even more attention to left handed writers is given in Improve Your Handwriting, by Rosemary Sassoon. Neither are very expensive but both offer excellent penmanship advice for contemporary italic writing styles. If you are hoping to improve a more traditional handwriting style, say a Palmer Method style, then Michael Sull's The Art of Cursive Penmanship is very helpful with some instruction for lefties (though the book itself is wire bound which, for a lefty, gets in the way of using many of the practice lines on the pages). That is available at Goulet Pens dot com for about $20 US plus shipping.
Improve Your Handwriting is good at continual reference to lefties through the text, but most other books include a couple of paragraphs at the beginning about proper pen grip and paper orientation and then never come back to discussing the unique challenges lefties face pushing a pen nib, connecting letters, crossing t's, etc.
There is also a small book by Vance Studley written in the '70s called Left-Handed Calligraphy, which is exclusively written for lefties. Still in print for about $7 US, and full of examples of many classic broad nib alphabet styles.
You might also look for YouTube videos by John DeCollibus, who is an accomplished left handed calligrapher.
Good luck, improving your penmanship is a totally worthwhile endeavor! Be patient and stick to it, the old brain/hand habits take a while to undo and retrain.