This is a quick video tutorial from the Medo Tech YouTube-channel.
You can simply get a cable and an adapter piece to your smartphone, then download an app, turn the live view on in your DSLR and there you have it! Your phone will work as your flip screen.
While the video refers to Canon DSLR’s and Android phones, the app is now also working for Nikon, and on iPhones. However, this particular app is no longer free, but I was able to find other similar apps that were.