You could use the top down example project by Epic as a base. What you would change is that when you click a location to move to, you would convert the world location to a grid index using the ConvertLocationToIndex3D function in BP_GridManager. Then use this index to get the tile location through the GridLocations TMap in BP_GridManager and make this the destination of the top down movement. For later converting from real time to turn based movement after combat starts take a look at the Hybrid example map in Maps/Example/Hybrid.
The toolkit procedurally generates grid locations and walkability. However, if you use a premade map you will likely need to do some moderate adjustments. Firstly, you should make sure the map conforms to a grid of the size you are using (200*200UU is default for the toolkit, but can be changed). If not you can get strange results like a tile where the center is on the very edge of a cliff, with half the tile hanging over thin air. Also, you will want to play around with collisions a bit. Anything that blocks the PathTrace channel will be walkable. Anything blocking RangeTrace will block line of sight. If bTraceForWalls is enabled in BP_GridManager, anything blocking WallTrace will block movement between tiles.
You will want to set up collisions appropriately for your meshes following these rules. You might also have to modify collision geometry for some meshes if they overlap tiles in weird ways, but that is on a case by case level