So the top tier goes on a board the same size as the cake, then buttercream/ganache then cover in fondant. Use dowels or bubble straws to support the bottom tier. Indent a board the same size as the top tier on the bottom tier and put your dowels within that. Usually a central one and 3-4 others. Trim to just above the fondant. Splodge some royal icing on top to secure. There's lots more detail re levelling cakes but ill assume you know that.