when is the best time of year to prune my magnolia and any tips on how to prune?
Vendée, Western France
It depends on whether it is evergreen or deciduous really as they respond differently and if you get it wrong they can react by producing water shoots - thin spindly things that sap energy and don't look good.
A small town not far from here prunes some of its street magnolias - evergreens - into formal cone shapes but I've never seen them actually doing it nor seen any flowers on them. I did have a deciduous magnolia here but it was already sick when we moved here. I trimmed off dead wood but it didn't like that either and I don't know whether it was already too late to save it or whether I hurried its death.
I suggest you have a read of this info from the RHS website as a starting point. Scroll down for pruning advice:
"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw