Here are 8 easy steps that lead you to teach Siri on your iPhone 4S to tweet.
1. In your Mac or PC browser version of Twitter, you must first enable Twitter text messaging (if you've already done this, you can skip to step 5). Do this by clicking on the drop-down list on the top right (next to your twitter handle) and selecting Settings. Keep in mind that if you don't have unlimited texting, this could get expensive.
2. Next, click the Mobile tab, third one from the left.
3. Enter your iPhone 4S mobile number with area code in that field labeled "Phone number," and then click Start.
4. You'll see this screen next, telling you it's waiting for you to verify your phone. (I've blurred the phone number for privacy) Pick up your iPhone 4S and move on to the next step.
5. Go to the Messages app on your iPhone 4S, and send a text to the number 40404 (or if you're not in the U.S., look up your twitter shortcode here), with your text message consisting of simply the word "go" without the quotes.
6. Go to your Contacts app on the iPhone 4S. Make a new contact by tapping the + sign, and name it as if its first name is Twitter. In the phone number field, type 40404. It will look like 404-04, which is fine.
7. Now you can start Siri as you normally do (by either holding down the home button on the iPhone 4S for a couple of seconds or just putting the phone up to your ear). Say "Text Twitter, and then tell it whatever you would like to tweet. Siri will read it back to you, and if it's correct, when she asks "ready to send it?, say yes. If Siri has misunderstood what you said, you can edit your tweet on the iPhone 4S screen by hand and then send it.
8. You can also have Siri read your tweets to you if you enable message notifications in the Twitter browser app. When you've finished making your modifications, click Save. You're done!