24 hours before, mix the mayo and dressing mix in a small bowl. Make sure it's mixed completely. Refrigerate until you're ready to use it.
On the day you're going to be serving it, peel and slice the cucumber. Thickness is a personal preference. I like mine about the thickness in the picture, or even a little thicker.
Put your mayo mixture into a zipper bag and snip a corner off (about the size of a nickel).
Squirt a dollop of mayo mixture onto each slice of bread. I usually squirt about a tablespoon or so, but it's not rocket science. Just squirt some! Use the cucumber slice to "smush" it onto the bread so you don't have to spread it with a knife. This saves a LOT of time.
The number of cucumbers you'll need depends upon how big they are (too big and you'll lose flavor) and how thick you like them. I always like to buy two medium sized ones, just to make sure I have enough. Leftover ingredients are great for breakfast! No kidding!