Hi dudleycat,
The Mimosa tree, Silver Wattle or more correctly (Acacia dealbata) can take several years before it flowers but this will depend a lot on where abouts in the country you are and the location it is in, in the garden.
It originates from new south wales,Australia and will do best in a warm sunny spot like a south facing wall in the UK. As the tree matures it will lose the Mimosa-like leaves (usually after flowering) and larger, simple leaves will appear. It flowers in January and is best pruned soon after flowering, making sure no frost is forcast.
To keep the tree at 12ft it will need to be kept pruned regularly otherwise it can reach twice or thee times that height. Good Luck. Tbird+
Ps I've just noticed, you said (how) not when to prune. Use a good pair of secautre's for finger thickness or Loppers for thumb thickness or a pruning saw for any larger branches. Cut back to the required size or slightly lower and new growth should sprout from dormant buds below the bark. Remove all dead, damaged and diseased wood.