If you're not home and need to be to sign for a delivery then the FedEx delivery driver will or should leave a door tag on your door indicating the delivery attempt and on that door tag there should be a space where you can sign the door tag giving permission to the delivery driver to leave the package.
Signing that door tag will work the same as signing your signature when the delivery driver shows up to have you sign for the package.
Just leaving a note will not work you'll have to sign the door tag in order for the FedEx delivery driver to leave the package.