Yes if it is well rotted you can dig in now and then plant your seeds in April

Thank you both, that is very helpful. I now have some well-rotted horse manure. Can I leave it in the garage until I use it or should I leave it outside in case it dries up in the garage?

If its bagged up it shouldn't dry out and in any case it won't hurt it if it does.

Get it dug in as soon as possible though if you plan on sowing seeds in April.

Bastard trenching is needed. Dig a trench across the plot,
about 10" deep, then half fill it with some of your manure.
Shredded newspaper can be added on top to conserve moisture. The printers' ink deters soil pests.
Then take out the next trench and put that soil on top of the manure/newspaper mixture.
When you get to the path turn sideways and chop out any couch grass roots that are spreading onto the plot, this
saves work later on and creates a drainage ditch at the same time.
Continue like this working backwards.
If you do this properly, when the ground dries out it should be possible to carefully rake over the surface with a 3 pronged cultivator to break up the lumps, and then a rake, to create a fine tilth for seeds.

Thanks for these comments. I intend to get the manure dug in as soon as I can - hopefully very early March. Trenching sounds like hard work but thanks for the information.

Trenching is hard work but it ensures that the ground is well dug over.