UPDATE: These instructions now apply to TweetDeck as well! You can find the field at Settings -> Services and pick “Other” for the shortening service.
So how do you go about setting it up you ask? Start by going to your YOURLS admin area and clicking the “Tools” link at the top. About half way down the page will be your signature (it’s like a password). It will be a 10 character string. Hang onto this, you’ll need it soon.
Now open up Twitter for iPhone and go to the “Accounts & Settings” screen. You can get there by pressing the three dots in the bottom right and then scrolling to the very bottom of the screen that comes up and pressing the “Accounts & Settings” button. There will then be a “Settings” button in the bottom left. There in the Settings, select “Services” and then “URL Shortening”. Lastly pick the “Custom…” option.
Now to enter the URL to your YOURLS API script. I found it easiest to craft this URL on my computer and then e-mail myself the URL which I could then copy/paste on my iPod Touch. It was a lot easier than typing it all out on the little keyboard.
This is the URL you will need to use:
Replace “YOURSIGNATURE” with your signature that you located earlier in this post and leave the rest alone (the “%@” is the placeholder that Twitter for iPhone will look for). If you e-mail yourself the URL, make sure that you still have “%@” at the end of the URL and not “%25@” or something.
Now when writing a tweet, press the character count button and the keyboard will disappear. There in the bottom right will be a button to shorten all URLs in your tweet. They’ll be shortened using your custom domain!