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.
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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:
https://www.rhs.org.uk/plants/popular/magnolia/growing-guide