You have to use the SEGMENT block, specifying the text that you want to chop, where you want to chop it and for how long.
For example, if you want the first character of a text, you should do this

This will return "T"
Now if you want the last character, How would you do it?
Hint: use the LENGTH block as one of the parameters of SEGMENT

If you get that, then you will also know how to get the text in the middle.