How do I power my track when using a Legacy system

No different than before - you just need a transformer that can supply a constant 18v directly to the track. If you don't have a transformer that has a built in voltage output setting or display, you can check the track with a voltmeter to be sure you're getting 18v and then mark that spot on the transformer. 

The Legacy system only provides the Legacy signal to the layout. In common parlance, when the Legacy signal is hooked up, it "overlays" the track power.

Thanks for responding Richie. I currently use the Lionel 18V Conventional Transformer (6-14198) to power my track and trains. Can I still use that? If so, wouldn’t it also power my train because I don’t want the transformer to control the train I want the Legacy Base/Remote to control the train. 

Today’s command control engines when powered up will first search for a command signal from your Legacy system. If no signal is present it will revert to conventional and respond to transformer input... when the engine detects the Legacy signal it goes to command mode and will respond to input from your Cab-2. 
Thats why you ALWAYS power up the Legacy base before applying power to the track.

Hope this helps...