If you want to speed up the process, some ironmongers and garden centres sell compost "starters" in powder form. The compost should generate a bit of heat so keep an eye on it and maybe thow a tarpaulin or similar over the bags if snow is about.

"It will take a few months but the volume will reduce and you should end up with decent compost."

Pile the leaves in a heap on the ground. They'll form leaf mould, which isn't the same as composted garden waste in terms of nutrition, but it's a very good soil conditioner. It will take months, and you can help by turning the heap every so often.

All good advice above. Here's my two penn'th

It will be rather a lengthy process with 20 bags, but last year I spread them over the lawn and then hoovered them up with the mower.

The additional shredding they get with this method reduces the overall time it would take to compost them ordinarily.

Make a leaf cage with 4 lengths of 2x2 and some chicken wire if you have room. It saves messing with all the bags.

However, as already pointed out, leaf mould is not compost, but you could add them to your compost heap (in conservative quantities) and give them a good mix in amongst the recently added garden waste (rather than in with the compost that might already be ready).

It takes between 1 and 3 years to make decent leaf mould due to the decaying times of the different leaves.

Some useful advice in these links

Covergirl

I thought I was the only one who used a lawnmower to collect and shred the leaves. I also 'Mow' my rose branches and BlackBerry branches to make them more compact and mix them approx 50/50 with normal garden (grass etc.) waste.You can also put a small amount of shredded paper (all you old bills!) in the compost and they will all rot down together.Instead of using a compost accelerator, just collect a few worms from the garden and drop them in the compost, the worms will thrive and make short work of your waste.