Ideally, this should be an ongoing thing. A good layer (4") of mushroom compost or some sort of organic soil improver should be added every autumn for 3 to 4 years. I just leave mine on the top of the soil to break down naturally but digging it into the soil would be far better.

A spring feed of growmore around plants would also help.

Even a mulch of lawn mowings is better than nothing if getting mushroom compost for the whole garden is a bit expensive, though you might need to add a bit of nitrogenous fertilizer in the spring as the rotting process can deplete it a bit. You can also chop up autumn leaves with your mower and use those as a mulch - the blackbirds will thank you!

Basically, anything organic may be used to feed your soil, from well rotted manure through home made compost to Council made compost. Either dig it in or spread on top and, as said,leave the worms to take it in.