Using a botanical press

If you're collecting flowering plants (including grasses) or ferns, you'll need to press your specimens before you mount them for your collection. Not only will it flatten them, it'll dry them out - preserving them from mouldering and rotting. Well preserved specimens will last long after your handing-in date: the oldest surviving herbarium collection is Italian and dates to the 1530s. Here's a carnivorous pitcher plant that was collected in the 1890s - it's the 'type specimen' for its species (the exact specimen that was collected and found to be a new species, and was then described and named).

The lowest-tech way is to bring your specimens home and then flatten them under a big pile of books and newspaper. It's better to get pressing as soon as possible before your specimens start looking wilted and sad.

You should leave a little label in with each one to remind you where you got it and what you identified it as. It's better to write a label because the other important thing to do is regularly change the newspaper. If you write on the newspaper you might forget to transfer the details across!

If you can, to aid drying, you could sandwich the plants in between layers of blotting paper as well, with newspaper layers between each blotting-papered specimen. You just change the newspaper, rather than the blotting paper. You can also put in layers of corrugated card in an effort to get the moisture out.

With most reasonably dry specimens, just newspaper should be ok though. But don't just abandon them - you do need to change the paper especially at the beginning, or your specimens could go mouldy.

When you first put them in, they might be unruly, but try to arrange the leaves, stems and flowers neatly, and not twisted / overlapping each other. It can be difficult, but if it's not exactly right, you'll get an opportunity to adjust them when you change the newspaper for the first time (their flatness often means they are more cooperative then) - do this change only one or two days after you first put them in. After that, how often you change the newspaper depends on how succulent your plant is - the wetter it is, the more often you'll need to do it and the longer it'll take to dry - check it every couple of days to start with, but if it's looking fine, then leave it longer. They'll be sufficiently dry for mounting in a matter of days or weeks, depending on size - you'll have to check and judge.

If you've got a specimen that's longer than your press, you can carefully zig-zag it. This especially applies to many grasses and ferns - though it's useful for ferns as it allows you to show the sori on the underside of a frond.

If you have a plant that drops its seeds, you can put them in a little envelope and keep them in the press. Extra large flower heads or pods can even be cut in half to help them dry.

If you want to be a bit more professional, you can make a field press. You can buy them, but I think it would pain me as they can be absurdly expensive, and are often not very big.

What you need (believe it or not) are two large cake drying racks like these:

They'll be the outer layers - the gaps help moisture escape. Put a layer of corrugated cardboard as the next layers in, and then your stacked layers of:
newspaper
blotting paper
plant specimen
blotting paper
newspaper.
You can also put some cardboard here and there between those layer groups, to help wick out moisture.

Now you need something to hold the whole creation together - you need two adjustable straps that you can pull very tight (VERY tight, use your knee or foot to brace yourself a bit!) - some sort of strap with an adjustable buckle. Canvas belts might do it or luggage straps. Space them out so they're squashing everything as evenly as possible.

When everything's secured, you can store it somewhere gently warm, like next to a radiator. I've read that people who live in hot climates recommend you attach your press to your car roof and drive it around like that to help dry the specimens. This may not be the best course of action in Britain. But it does remind you that you can take your press into the field and add specimens to it as you walk around - this can be best because they have the minimum time to wilt.

You can see lots of home-made presses in this google search, so do feel encouraged to have a go. But realistically, sticking specimens under a pile of books isn't a bad option. Don't forget to change the newspaper though!

I've also made some suggestions on how to mount your pressed specimens. 
If you're collecting large ferns you might want to fold some over so they take up less space - you'll want to show the sori on the underside in any case.