It's a very easy and simple process to schedule tweet threads.
In the Tweet Text box of the New Twitter Tweet form, enter all the tweets in the text box and separate them with the string [[tweet]].
For example:
This is tweet #1.
[[tweet]]
This is tweet #2.
[[tweet]]
This is tweet #3.All the tweets in the thread will be published at the same time.
Any uploaded media will be attached to the first tweet of the thread.