Icing sugar woes

[FlourishandBlotts]

I'm quite a novice when it comes to baking and icing cakes.

The past few times I've tried to ice my cakes using silver spoon icing sugar... however...I either end up with an un-spreadable lump of icing or a mixture that is too runny!
Does anyone have any foolproof ways to get a nice thick but able to pour icing?!

I don't often use 'runny' icing - I tend to use buttercream or ready-roll.
When I have used 'runny' icing, I make it in small quantities, with warm water.
Make some up extra-thick, put it into a piping bag with a small, round nozzle. Pipe a fine line around the edge with this thick icing and let it set. Now make a batch slightly thinner/runnier and pour that into the centre. The piped edge should contain it all.

They make a fondant icing sugar in a green box instead of pink, which I guess would have a smoother consistency, but tbh for covering a whole cake I'd be using fondant or buttercream.

Water icing is ok for popping a drizzle on the top of fairy cakes or eg on a coffee cake with coffee buttercream inside, but then it still comes down to getting the right consistency, not too solid but not too runny. Not sure what the answer is? For chocolate cakes I always use ganache now for a lovely shiny topping. 100ml single cream heated with 300g of broken up mixed dark & milk chocolate stirred in. Lush.