Try this, make a roux with just butter and flour, make sure there are no lumps and it should be smooth. slowly add your stock so that you won't get flour lumps, let that thicken, then add your milk, again slowly. whisking/stiring the whole time. If your gravy still isn't the texture you want make a cornstarch slurry, its just cornstarch and water. Add that a little at a time and that should thicken it. A slurry doesn't have exact measurements, its just put cornstarch in a bowl and water till its dissolved, it doesn't take a whole lot of water.... hope this helps...
