I assume you mean the Samsung Here maps app on the watch. You need to install Here maps on your phone first. And note that there are two watch apps: one called "Maps" and one called "Navigator." Make sure Navigator is installed on your watch; that's the one that actually delivers the turn-by-turn.
Then the most reliable way to do it is to set up a route on the phone app, and then when you start navigation on the phone, with luck your watch will start providing turn by turn instructions, buzzing as you approach each turn. If the watch navigation does not start automatically, try running the Here navigation app on the watch to see if it connects to the phone.
In my experience, it sometimes takes a few tries to establish the connection. And in theory you can initiate navigation entirely from the watch (causing the here.app on your phone to load and start providing navigation; the smarts is still all on the phone). But I've found it more reliable to get the navigation started on my phone before I leave the house, and then let the watch take over once I'm out.