That used to be in as a default feature, actually, and I’ll make sure to add it back in. Here are some quick steps to add it to the current version:
Make a copy of any of the decals used for markers, such as Decal_CurrentMarkerHex. Lets call it M_ArrowDecal. Replace the texture sample with the arrow texture included in the toolkit.
Go To BP_GridManager and the DisplayPathAsInstancedMeshes function. At the right hand of the function, right before meshes are normally spawned, add a branch checking the bUseDecals boolean. If it is true, spawn a decal instead, and add it to a new array of decals that we make, like so (note the values in the Spawn Decal at Location node):
Then, in the DestroyAndClearSplinePath function, add the following:
Then, add this function at the very start of the DisplayPathAsInstancedMeshes function and in ATBTT_PlayerController in the RemoveDisplayedPath event, ignore the bDisplayPathAsSpline branch and always run the DestroyAndClearSplinePath function instead. Like so:
That should do the trick. I’ll add this back in more elegantly in the update after the next.
Happy to hear that 
Make sure to learn about UE4 multiplayer implementation from Epic’s tutorials beforehand. Even though everything will be replicated in the next update, when you’re adding your own features you’ll need to know how.
Go to ATBTT_GameMode and replace the value at SetActorLocationAndRotation of BP_GridCamera_Ref to whatever you want:
