Secure the 6-inch cake drum to the 8-inch cake board (if using), using paper adhesive stickers (or other similar, temporary adhesive). Set on a turntable, with a layer of non-slip fabric in-between the turntable and the boards.
Spread a dollop of icing onto the cake drum, then set the first, cooled cake layer on top, making sure it is level and centred on the drum. It should line up with the sides of the drum.
Pipe an even layer of lemon buttercream all over the surface of the cake layer, making sure to go right to the edges. Use an offset palette knife to spread the icing out evenly, ensuring there is some overhanging icing at the edges.
Pipe another circle of buttercream on top of that layer, just around the circumference of the circle. This will act as a buttercream 'dam' to keep the lemon curd filling contained.
Spread a thin layer of lemon curd on top of the buttercream, making sure not to go over the edges of the buttercream 'dam'.
Place the next layer of cake on top, making sure it is centred and level, then smooth off the excess icing around the sides and place the cake into the fridge to chill for 15 minutes before repeating the process all over again with the buttercream, curd and final cake layer. Place the whole cake back into the fridge for a further 15 minutes.
Add 4 cake dowels, if using. Add a splash of milk to the buttercream to thin it out slightly, then use to apply a thin crumb coat all over the top and sides of the cake. Place back into the fridge to chill for a final 15 minutes.
Apply a generous layer of icing all over the top and sides of the cake. At this stage, cover the sides of the cake drum with icing too. Smooth out the top, making sure to leave some overhanging icing all around the top edge, then use the icing smoother to smooth out the sides, moving the turntable, rather than the smoother for best results. The overhanging icing should be pushed upwards and inwards during this process, forming a nice 'lip' around the top of the cake.
If needed, apply more icing to any uneven sections and smooth again.
Once happy with the finished cake, set aside at cool room temperature to firm up, until ready to eat/transport.