How do I gather cutains?

[Eddas]

Please would someone tell me how to gather curtains?! I have bought some new ones for the front room and they are pencil pleat ones. In all other rooms I have tab-tops so i've never had to gather them before!

I assume I need to pull the strings that go along the width of the curtain? should I pull all 3? Do i pull from each end?> why is such a simple thing so complicated

TIA

curtains are so complicated.
My mum alays says to me ' well you just need to measure it and double it for gathering.'
Ok but do I double it for each curtian.
Dunelm brings me out in a cold sweat!