Measure the height of the tile and the height of the adjoining floor. This is the amount of reducing you need to accomplish.
Measure the length of the tile edge that you want to reduce -- usually the door frame width. This is the length of the threshold you need to purchase.
Decide what type of material you want the threshold to be. Most often, the choice is marble, but you can also use wood. Purchase the reducing tile saddle that's the right length to fit the transition space. If possible, have the retailer cut the piece to fit -- just be sure your measurement is perfect.
Set the threshold in place, leaving the high end to sit against the edge of the tile. Spread construction adhesive on the bottom of the saddle and on the tile side. Press it into place firmly. Wipe away any excess adhesive with a dry rag.