Hi clair
You can buy travellers cheques from your bank (sometimes for no commission) or from travel agents, post offices or even Marks and Spencer in various denominations.
Make sure you get sterling travellers cheques - it's normally not a problem to get them straight over the counter, but I normally order mine a few days beforehand - just saves a lot of faffing around.
You will be asked to sign them in front of the cashier and given a receipt with all the serial numbers and on which must be kept separte from the cheques - that way if they're lost or stolen you can ring a helpline number (details supplied with cheques) and they will be replaced in a matter of hours.
When cashing cheques in
Icmeler - you will need to sign the cheque again in front of the person cashing. If you use a bank or change office you will need to take your passport with you. I normally use a supermarket or shop (as they don't charge a commission for changing) and you just need to write your passport number on the reverse of the cheque.
I normally take most of my spends in travellers cheques as it's just safer, and I know if they were lost or stolen my holiday wouldn't be ruined as they are quickly replaced.
Hope that helps!
Mazz