When my mom makes frosting she just dumps a load of butter in a bowl, mixes it until it's soft, dumps in a box of icing sugar, and adds vanilla to taste. 
That's more or less what I do, with some dribbles of milk added in -- hence the simple recipe I posted earlier (in case she wanted more precise measurements).
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Frosting out of a can will be easiest, yes -- but may not have the taste you are looking for. The Betty Crocker tubs of frosting will be the same as the Betty Crocker tubs of frosting in the US to be sure -- so yummy spread between graham crackers which I wish we could buy here!
If you want something fresher tasting & less preservative-laden, then I'd mix my own frosting.
The box mix for the cake will have all the directions on the side of the box -- these usually call for adding (to the mix) egg(s), oil, and maybe water or milk. It will tell you how long & how hard (what setting) to mix it with an electric mixer.
I would like a photo too or at least a report of how you come out with it. Mmmmmm - I like cake!