I don't know about elm bast, but assuming it works the same as Lime bast, i processed a load a few years ago.
I stripped the bark off in narrow strips (<1") having first scraped off the outer bark as far as i could. Them half of it I tied loosely in a bundle and left in a stream for a week (as per some books) and the other half i put into smaller bundles and rolled them into loose coils (tied in place with string) then I boiled the coils in a woodash and water solution for a few hours.
The stream stuff was partly retted and stank, also lost soome of it downstream. I composted this lot in the end
The boiled in lye was all good.
I took it out and rinsed off thoroughly in the stream to remove the lye, then hung it up to dry. some of it got left to dry out over night but most of it i rubbed between my hands to seperate the fibres and remove the last of hte outer bark that i had missed.
The dried out bundles were a little more difficult ot seperate the fibres from, but not impossible. The still wet fibres came away freely. Oh and rewetting the dry ones didn't help
The reason I did this was that during my MA (Experimental Archaeology) I carried out some breaking strain experiments with a variety of natureal fibre cordages. Lime bast, honeysuckle bark and sedge. choices were dictated by what was about on campus at the time, otherwise stinging nettle would've been there too.