1. Bookmark any page, making a dummy
2. Menu > Bookmarks > edit
3. Change URL of dummy bookmark to the js bookmarklet code.
4. Visit any site.
5. Menu > Bookmarks > tap on the bookmarklet
6. Widget appears on bottom right of page
It doesn't work on HN(?) But does work on other sites.
5. Tap on the URL bar
6. Type part of the name of the bookmark you chose until it appears in search (in my case eruda works)
7. Tap the bookmarlet
For this to work you need to have bookmark search enabled in settings: Settings -> Search -> Search bookmarks
Also, there seem to be many sites where the widget doesn't appear, but you can try it at google.com.
It helps with things like removing elements because you can see the DOM and it's fewer clicks away and easier to use than ublock, which doesn't show the DOM in the little box provided for element removal and only allows removing one item say a time (you can use multiple selectors, but every time you tap an element to get the selector it overwrites the existing content)