Sleeping bags come with "stuff bags". I am the stuff bag goddess in this house.
Lay your bag out as flat as poss. Don't fold in half completely - fold in half from the top (the bit where your head is), but let the fold peter out towards the bottom. Then roll up from the bottom as tight as you can. Once you've rolled up to the top, insert into stuff bag. It WILL spring open - but that's when you start stuffing. And you stuff anything in which way you can. Just keep stuffing til it all goes in.
An expert can do this in about a minute!