mine also cost £6 for blackheith market
and way fill it about 3/4 full of water and put some flavoring in eg. strawberry then simply put the amount of pellets you require in (not to many at once) then place the lid on and pump about 7-10 times then release the air ( there should be a button) Dont worry if the pellets don't sink just repete the proses and they will sink eventually they will now be soft and will not float ow and of course have you flavor on them
the pellets you want to buy are callled "expanders"
and what ever you do dont get any water in to the lid is is a pain to get out that is if yours is like mine..