To indicate that a letter includes an enclosure, simply write the word 'Enclosure' below the signature line. It's helpful, particularly if there are multiple enclosures, to follow this with a list of the items enclosed.
The simplest way to do this that I know is to leave two or three lines after your signature and then just write "Enc" You can put the number of enclosures after this, for example Enc(2). It would look like this:
[body of letter]
Mr A Smith
There are 4 four methods: The first one is to write it on the envelope outside. The second one is to write it inside on the paper. The third one combines number one and two, the foruth one is to send a separate fax.