The stamps you have are fine.
You need to focus on International rules only. Those sizes and general info can be found here:
http://www.royalmail.com/personal/inter ... l-standard
All you need to do is get the corresponding
International price for your item and use enough stamps to cover it

You can use any stamps including the ones you have.
It is that simple I promise.
Write out your SASE
Use the Royal Mail calculator to get your price.
http://www.royalmail.com/price-finder
Use a normal calculator to see how many 1st class stamps needed to meet/exceed that price- each stamp you have is worth £0.63.
Stick that number of stamps on your SASE.
DONE!
