The easiest way to schedule tweets is using tweetdeck. You can access tweetdeck online at tweetdeck.twitter.com

This is the screen you’ll first see, which is to show how tweetdeck looks. Unlike normal twitter, tweetdeck has columns for each page you’d have on normal twitter (timeline, notifications, messages).

So you can start by clicking on the blue quill in the top left corner

A new column will appear into which you can write your tweet

When you’re happy with your tweet, click on schedule tweet

A calendar will appear. Choose the date and then enter the time you wish the tweet to be sent at

When you’re happy with the time, click on the “Tweet at” button which should have the time you’ve chosen as part of it

Once the tweet is “scheduled” a new column will appear in tweetdeck called “Scheduled”

And when the time comes, the tweet will be sent, and will disappear from the “scheduled” column