A wye, or any reversing track structure like a loop, ultimately connects the left rail to the right rail. You'll need to insulate somewhere, and have multiple feeders with polarity-reversing toggles, but exactly where can depend on the switches you're using, as they may be insulated at the frog, or not (some even insulate the outer rail). There's also more than one way to wire up a DC reversing wye.
You can visualize this if you draw the wye, and label the two rails of the main + and - (I'm assuming the wye diverges from a main track), then trace them back. Both outer rails of the diverging part of the wye end up being the same polarity. If, for example, both were positive, then tracing them back you can see both rails beyond the third switch need to be positive, which obviously wouldn't work.
The simplest (I think) approach involves adding a DPDT toggle wired to reverse polarity between the power pack and the track. Then gap all four rails at the frog end of one switch (the non-mainline switch), wire the output of the toggle on the point end of the gapped switch, and the direct output of the power pack to two or more places on the other side of the gaps (two or more because it depends on where the track is insulated; at a minimum on the point end of both other switches, and possibly in other places).
If the switches you're using don't insulate their frog rails, you may also need to gap the frog rails at the other switches to avoid a thrown switch causing a short, and then that will make it necessary to add some redundant feeders (direct, not through the toggle) so the isolated sections of rail get power.
See
this page for a diagram that's similar to this (just omit the toggle for the main track as it's not needed with DC).
When using this, you set the toggle to match the left side of the wye, run the train up the left side and through the end switch, throw the toggle to match the right side, then reverse direction on the power pack and run down the other leg of the wye.
The NMRA describes a slightly different approach
on this page.
Wye's are a pain to wire, even on DC.
