It's very common for people to snatch one of those diagrams off the Web, and say their wiring is exactly like that. Probably not "exactly".
The first task is to identify the travelers. Wire colors in 3-way circuits can be anything. Each of the old 3-way switches will have 2 brass screws and 1 black screw. I strongly recommend color-coding in situations like this, and to mark the traveler wires with yellow tape. Both travelers can be marked the same since they are interchangeable. Do this on both switches; they should be the same pair of colors on both ends.
If you've already dismantled the old 3-ways, put them back on temporarily, wired so they work absolutely 100% on both ends. When they do, mark the travelers. Don't shortcut this process or make assumptions; they will blow up in your face later.
Now look at the third wire that is in the cable with the travelers. Is it white? Does it go only to other white wires? If so leave it alone. Does it go to a switch or to any black wires? If so, re-mark that white wire with black tape on both ends -- this mark is required by law. If you know for a fact that it is switched-hot (hot only when the light is on), I prefer to mark it with red tape.
Now, with those marked, look at any white wires that remain. Do any of them go to switches or to black wires? If so, then Houston, we have a problem. We will not be able to install this switch on this 3-way complex. Stop entirely; you cannot continue. You will need to pull additional cable, post pictures of the insides of your boxes and we can advise.
Ok, so now all travelers are marked, as are any white non-neutral wires. We know we have power coming through a 3-way box, because white wires remain. At this point, this is straightforward.
On the Wemo, mark the red and blue "traveler" wires with yellow tape.
- Add green to existing bare or green ground wires.
- Add white to existing white neutral wires.
- pull the 2 yellow travelers off the old 3-way and attach it to the yellow wires on the Wemo. Which you re-marked.
- one wire remains on the old 3-way. Remove it and attach it to Wemo black.