Stage 1 - 10 min.

Start by washing 3 kgs fresh spinach in several changes of lukewarm water (not too cold, like for all earthy vegetables).
Stage 2 - 2 min.

Drain.
Stage 3

To prepare the leaves, you need to remove the central vein, for that there are two methods:
Fold the leaf in two along the central vein.
Stage 4

Then with a knife, remove the central vein.
Stage 5

You should keep only the green and tender part of the leaf.
Stage 6

Second method: fold the leaf in half along the central vein.
Stage 7

Then detach the vein by pulling on the stalk, from the base towards the top (in the direction of the arrow), until it's completely detached.
Stage 8 - 30 min.

Stage 9 - 10 min.

To blanch spinach
If you wish to blanch before cooking, which brings out the flavour, boil a large pan of water, then salt using 1 tablespoon of salt per litre of water.
Also prepare a large container, or your sink, full of water as cold as possible (ice-cold).
Stage 10

Tip in spinach leaves.
Stage 11 - 1 min.

Plunge the leaves down under the water, while waiting for it to come back to the boil.
Stage 12

As soon as it boils again, remove spinach from pan with a
skimmer, and transfer to ice-cold water.
Stage 13 - 5 min.

As soon as spinach is completely cooled, drain thoroughly.
Stage 14 - 5 min.

Then use a salad spinner to remove as much water as possible.
Note: you will sometimes read in other recipes that you should squeeze spinach in your hands to remove water. I don't think this is a good idea because it gives you hard blocks of spinach, with the leaves all stuck together, not so easy to use.
Stage 15

And finally set aside on absorbant paper to collect any remaining water.
The spinach is ready to be used in your recipe.
Stage 16
See how to in this video.
Stage 17

If you wish to eat it like this (and it is already excellent), you just need to heat it through in a frying pan with a knob of butter.
Stage 18 - 1 min.

Spinach "wilted" in butter
If you have not blanched you spinach, take a high-sided pan, and melt 60 g
butter in it.
Stage 19 - 5 min.

As soon as it's fairly hot, add the spinach. If it won't all fit in the pan, wait until the first batch wilts, then add the rest.
Stage 20

As soon as spinach wilts, salt and pepper, taste to check, it's ready.