This was something that took me quite a while to perfect, but trust me, once you’ve got your buttercream recipe down, its well worth it!
My recipe has always been 1:2, so for example 4oz butter and 8oz icing sugar. One of my little tips is too warm the butter so its nice and soft (but not melted) in the microwave before adding your icing sugar. This helps to keep it lovely and shiny with no lumps.
If its a bit stiff, another tip is too add a splash of cream or milk whilst mixing! This leaves you with a lovely pale and smooth buttercream.