What about it ?

It doesn't need to be pruned as that has no real beneficial effect but you could do a small amount - maybe just a few inches - after flowering.

Pruning should be done immediately after flowering, preferably yearly once they're over 3 years old - they flower on new wood grown the summer before, so the later you prune, the fewer flowers you'll have the following year. More info required if you have any other specific queries relating to pruning.

If you cut some of the branches with flower buds around Christmas time and put them in water in a cool room, and then when the decorations are down bring them into the warm you will have forsythia flowers a good month before they are out in the garden. And that's some of your pruning done for you! As Bamboo says, cut off all the flowered branches as soon as the flowers fade. If you stick some of them in the ground most will root. I accidentally got some cuttings when I used my prunings as pea sticks!