Collecting seaweeds

Gaugin's 'Seaweed Gatherers'. You probably don't need this much.

I wasn't actually around in the Victorian era, but I am a fan of the writings of the Victorian naturalists. They're often full of very practical advice. I'll refer you to W H Grattan (writing in 1872) for some tips:

At once then, to the shore, but not to the sandy shore, for only useless decayed rubbish, or here and there some straggling Zostera marina will be met with. The collector must away to the rocks, and search carefully every pool he meets with, from a little distance below high-water mark, and so on down to the water's edge, always remembering that it is better to collect while the tide is receding than as it is coming in.

He describes how the red species will be found mostly on the lower shore - they are adapted to the lower light levels - but that you may also find them growing on the stems of kelps that have been washed up. 

When you collect your specimens, you want to take enough of the thallus (the entire seaweed) to make sure you can identify it - include both the leafy bits - the blade, and the stemlike bit - the stipe. The Official Guidance for the assessment suggests you also include the holdfast, the rootlike bit that holds on. However, I've also read that you should definitely leave this because it will enable the plant to grow back.

My advice is to be sensible - nobody is going to mind if you take a piece of bladderwrack when the species is carpeting the beach. And ideally you should always seek the permission of the owner - they are likely to be perfectly happy to allow you and indeed may be very interested in your findings. I know - all those years of beachcombing as a child... you never asked for permission and it might seem absurd... but I'm just giving you the information so you know the official line. I've written a page about collecting on the seashore here.

While in the field, don't forget to make careful notes on the specimens' location and habitat. You'll need to know which one you got where - you could put them individually into labelled bags, or make some sort of label to tie on (they'll have to be waterproof, remember).

Back home, the specimens have to be attended to straight away. Grattan says:

Beginners should be cautioned against the error of bringing home too many plants at a time, or lose choice specimens which will decompose unless they are attended to before night. The first thing is to empty the bag into a basin of sea-water and select the best plants, giving each a good swill before placing it in another vessel of clean water. Get rid of rejected plants at once.

He then describes how to mount the specimens - I have written some advice here.