You are doing well so far!
This is the tricky part...
Edit the go.mod to have a line like this near the top
replace github.com/jlaffaye/ftp => /path/to/src/github.com/jlaffaye/ftp
That will tell go build to pick up the ftp module from the absolute path you provided. (You might be able to use relative paths &or env vars, but I always use absolute paths!).
That should build using the new ftp module you made.