1
Re: our meeting tomorrow.
To sign this email context, you can fingerspell R-E to represent the abbreviation for 'regarding'. Keep your hand steady and smoothly transition from the crossed fingers of the 'R' to the curled 'E' before signing 'our meeting tomorrow'.
2
I need to redo my homework.
When 're' is used as a prefix meaning to do something over, you generally do not fingerspell it. Instead, you convey the concept by signing the main action—in this case, 'do'—followed by the sign for 'again'.
3
Please rewrite the essay.
Just like with 'redo', the prefix 're-' is translated conceptually rather than literally. You would sign 'write' and then 'again'. Fingerspelling R-E here would be confusing and look like you are spelling an English word rather than using ASL.