This project is very early stages and is not yet usable as a desktop environment. However it is available for anyone to test, play with or contribute to.
Using standard go tools you can install Fyne's desktop using:
go get github.com/fyne-io/desktop
And you can run that simply as:
cd $GOPATH/src/github.com/fyne-io/desktop go run cmd/fynedesk/main.go
It should look like this: