On desktops I've used gforth[0]. It's rather large and heavyweight, but easy to install and has decent documentation; not a bad starting point. Alternatively, JonesForth[1] is an excellent extensively-commented implementation suitable for reading start-to-finish.

[0] https://www.gnu.org/software/gforth/

[1] https://github.com/nornagon/jonesforth/blob/master/jonesfort...