Hi Charlotte
To delete and replace a letter in a word, or a word in a sentence, you start with a correction in the text with a stroke through the letter or word, then you write the replacement letter or word in the margin followed by a separator stroke.
If however, you needed to just delete a letter in a word and not replace it with anything, and the gap needs to be closed up, then you use a delete and close-up mark. You use this correction to alert the typesetter that the deletion of a letter has created a gap that needs to be closed.
There is an explanation of both these corrections on page 37 of Tutorial 1.
You also need to use a delete and close-up correction when a deletion directly precedes a punctuation mark. There is an example of how this looks in line 3, Practice Exercise 4 on page 60 of Tutorial 1.
Hope this helps!
Val