You're not an idiot I was like this when I first wanted to send abroad. I have always put in 2 single dollar bills with my US fan mail and has worked so fair.

you gotta go to post office. They will sort it out for you. Don't just put it in post box.

It depends on what you send. If for instance you are sending a lor, 10x8 photo and a return addressed envelope this will cost you about £3.15 from the UK to the US. The post office doesn't supply IRC's anymore so you have a number of options.
1. Put $3 dollars in cash in the envelope to cover return postage
2. Buy US stamps. Search this forum and you will find people who deal in stamps. It costs about $2.90 to return the above example

Hope this assists

AJB1207

Right I might look a bit of an idiot here, but I am wanting to send from the uk abroad, e.g. America and Europe etc, and I am wondering about postage?
Do I put a large stamp on large envelope or more on staps or what?!
I am confused.
Your help is really appreciated. :-)