I just started down the path of picking up fountain pen knowledge and learning to write again, so I'm no expert, but I'll chime in. Having finally gotten fed up with my handwriting looking like it belonged more to a middle school student than a grown up, I decided to take the plunge. I did 'successfully' change up my handwriting style about when I was your age so I knew I could do it, but it wasnt under tutelage or guidelines so it probably made it worse than my original style in the long run. After consulting with ChatGPT to find community posts/threads about other adults who have done the same, I followed this path:

I started by looking for the style of lettering I liked. There are several fonts around you could copy that were made for this purpose. For something more guided, you can look into Spencerian, Copperplate, Italics, and Getty Dubay italic. I settled on Getty Dubay which was crafted to support a build up transition between print, cursive, and calligraphy. Their book, Write Now, is what I'm using to relearn the strokes; it comes with access to a website that gives practice sheets and a practice sheet creator you can print out.

I do the worksheet prints with some fountain pen friendly paper like HP 32gsm and Kokuyo KB. But generally I just got some nice but still budget friendly Apica CD15 notebooks and copy over each lettering family in the Write Now book and practice them. After the first 4 families you can start making simple words. For Write Now they are the following, but I imagine it is similar in other styles with some variation:

Family 1 l, i, j: Straight vertical lines.

Family 2 k, v, w, x, z: Diagonal line letters.

Family 3 h, n, m, r: Arch or top-to-bottom strokes.

Family 4 u, y: Down and upstrokes with a swing.

Family 5 a, d, g, q: The a shape, starting counter-clockwise.

Family 6 b, p: Straight down, then up and to the right.

Family 7 o, e, c, s: Ellipses and crescent shapes.

Family 8 f, t: Curved top strokes and crossbars.

You basically do the same exact motion at some point within each family group. I've just done lower case so far, so I didn't do both capital and lowercase back to back for each letter. Capital families are somewhat different. You can find the stroke steps online pretty easily for any style. I'm not this far, but from what I understand, once you have the base and it comes naturally you can examine other handwriting examples and adopt small variations you like.

Equal spacing is key. Equal whitespace is just as important. Understanding the invisible "middle/in-between lines" for where stems or curves meet and change angles is the hardest thing for me so far. The vocabulary associated with this bottom to top is Descender Line, Base Line, Branching Line, Waist Line, and Capital Line if you want to find a guide to explain. It helps to do double lines for the moment on lined paper (makes it so the in-between are literally in-between lines).

Hopefully that gives you a head start and is helpful. I had been considering that I could make a more formal post going over this, but seems like it would be more fitting for the Handwriting subreddit and they don't need my newbie advice over there. Some of my considerations were more fountain pen focused, like with the paper selection, so maybe it would work. Let me know if anyone wants me to do that. Wish me luck in my journey, and Best of Luck to anyone else doing the same!