To use this system at all, two things must be true.
- Power must enter the 3-way complex at one of the switches, not the lamp. AND
- You must use one of the two available models of matching smart remotes, not a plain 3-way.
If either of those is false, you bought the wrong family of smart switch. Try one that uses wireless or powerline communication. This is the end of the answer for you.
Now, this family of switches has a master (4 wire + ground) and a remote (3 wire + ground). You must use both and You get no choice of which goes where.
Both 3-way switch boxes will have one cable that is /3; that is the colors are black white red. We will totally redefine how its colors are used, to accommodate the needs of the new smart switch. I need you to mark the red wires with alternating bands of yellow tape, so its color is red-yellow.
One of the boxes has the /2 cable going up to the lamp. It will be black and white. Mark the black wire solid red. This box is the one that must take the master smart switch. This box now has a red wire, a red/yellow wire, some number of blacks, and some number of whites. The master switch has those same exact colors, thanks to the magic of colored tape. Match 'em up. All blacks together under a wirenut, etc.
Now the remote. If you are using Leviton's "Coordinating Remote Switch", with 4 wires + ground, then you need to cap off and tuck back the red and black wires. You will not be using them and do not connect them in this next step. If it is Leviton's "Matching Remote Switch" (why are there two kinds??) then do not cap any wires, you will use them all.
The other box will now have a red/yellow, some number of blacks, and some number of whites. The smart switch remote goes here. Same deal.