Heathrow terminal 3 to terminal 5

Two tickets or one? (Connecting implies one, but best to check)

Checked bags or only carry on? (important if separate tickets)

Will your bags be checked thru to Milan or are we talking separate tickets?
Transfer between terminals is via a free underground train - pretty simple and efficient even with luggage - so the basic logistics and timing shouldn't be a problem unless you encounter some unforeseen delays.
There are videos on YouTube that can provide a basic tutorial of how the system works.

The transfer is by the Heathrow Express (FOC) if done landside. This is really to be avoided if possible as it involves immigration; the airside bus will be quicker.

The train is the Heathrow Express which will be free before it heads into London. Before going through the turnstile , go to a machine to the left to get a free ticket to allow you to pass through turnstile. Terminal 5 is a nice, new light filled space whereas Terminal 3 has seen better days, tired and worn facilities. You have a long enough layover if you don't have to pick up luggage and go through security again.

https://www.heathrow.com/flight-connections

This above may be helpful--- plug in your flight info

is this you flight plan?

https://www.heathrow.com/flight-connections/personal-connection-planner/VS104/BA562/01-JUN-2019/01-JUN-2019/5/7D

Whenever I've made this type of connection (usually American Airlines to a BA connection), it's been been through a quick document check leaving terminal 3, an airport bus transport to terminal 5, and process through security as if coming through from the "land" side (steel toe/shank shoes off, all electronic/liquids out of bags).

One friendly tip - if you are traveling with a companion, DO NOT put both bags of liquids/creams/pastes/jells in the same x-ray machine tray.

The Heathrow web site has detailed connecting flight transfer information...

E Gates are now open for US Passport holders (and others), you should get through that FAR quicker than before...

If your luggage is checked thru to Milan, or if you only have carryons, then follow the Connections signs as you exit the plane. This will lead you to airport buses to other terminals, including Terminal 5. I’ve found it very easy, even with jet lag. And there are Heathrow and airline staff along the way to help.