With some exceptions, pruning really is not widely considered a must (or even desirable) for this popular flowering tree. For one thing, magnolia trees do not heal as well as most trees from pruning cuts (which can lead to the introduction of diseases), and, for another, you can easily spoil the looks of a magnolia tree by getting carried away in pruning it. But if you still decide to prune, consider this advice:

Do so just after flowering is done, not only so you can at least enjoy the current year’s blooms, but also because magnolia trees bloom on old wood (which means that pruning too late in the year puts you at risk for losing the buds for next year's flowers).

Trim off only a small amount, with an eye to shaping the plant to your liking.

Because magnolia trees heal relatively poorly after you prune them, making them susceptible to diseases, also keep the following in mind if you feel that you must prune:

Make your cuts with pruners or loppers that have first been thoroughly disinfected (for example, with ethanol or isopropyl alcohol) and sharpened.

Cut off dead branches first. Next, inspect the plant for areas where two branches are rubbing against each other; decide which one will lend itself better to the overall shape you are aiming for, then remove the other one. Finally, prune off any limbs that you suspect are infected by a disease (you want to make sure that the disease does not spread). Disinfect your cutting tool once again after this part of the job is done and before continuing to prune.

Make clean cuts with your pruners. Prune all of the way through the wood. Do not leave a branch hanging "by a thread" and then attempt to yank it off with your hand. In the process, you could end up ripping some bark off the trunk.

Watering your tree is recommended during the summer drought season. It is important to water deeply, once a week.

Hope this helps!