How to start learning handwriting...?

For apps there’s Ringotan which is probably the most tightly integrated with WK and is still free afaik.

There’s also skritter which is more robust, but was a relatively expensive sub the last I looked at it.

Personally speaking, most of my writing practice was just using a cheapie composition notebook from the grocery store. A lot of the others I’ve seen have ordered paper specific for kanji/japanese, but if it’s really just the basics any practice is good. Kind of like how WK breaks things down into radicals after a while there’s something of a feel for writing kanji/stroke order without having to meticulously study all of them.

I know there were some people discussing more penmanship related topics on the forums here before and I hope they catch this thread. There’s a lot of stuff that just won’t show from stroke order diagrams. Speaking of stroke order diagrams both Jisho and JPDB show them on the kanji pages. There’s an open svg library out there as well, but that’s more for techies.