Easiest is to pour it into a sanitized jar. You'll want at least a 5oz./150ml jar, though a 12 oz. bottle can work just as well. I find that I prefer tall slim jars to short fat ones, though pretty much anything you have around should do the trick.
Swirl the yeast cake and whatever remaining mead you have in the primary so that the yeast is well mixed. Then pour it out through your spigot if you have one, or through the top of the bucket / carboy if you don't. Sanitation is important here, as always, though if you sanitized properly before fermentation and were careful not to contaminate anything whenever you opened the fermentor until now you should be fine.
After you have your jar full of yeast, label it and put it in the fridge. If you want to reuse it 20 times you have to be extremely careful about things like how slowly it cools down. If you're only going for another 2 or 3 batches that doesn't really matter. You can just stick it in the fridge and forget about it until you need it.
That yeast will keep for a LONG time. Months, definitely, and more likely over a year. However, if you let it sit for more than a couple of weeks you will definitely need to pitch it in a starter before using it in another large batch. If you reuse it within a week or two you can just pitch it as is. (You can actually just pitch it as is for a long time without doing a starter, only the yeast will crap out on you a lot earlier and you'll end up with a mead that isn't as good as it could have been.)
Storing the yeast in mead gives you an easy way to determine whether it's still good. The yeast will settle and the mead will stay on the top over time. When you open it, give it a smell and a taste, and you will know right away if there is an infection of any kind. If there is, don't use it. If there isn't, even if it's old you should be fine.