In order to put the flaps up, you will need something to stand on. Mounting blocks work very well but a shavings bag or hay bale works if that's all you have.
Stand inside the tent and look up at the edge. You should see something like this (I'm not showing the flap here), with a rope running along the inside edge of the tent between the end of the roof and the decorative border:
Adding in the flap, you will see that it is attached to the rope by a series of plastic clips. Depending on the age of the flap, there might be lots of clips or there might be many that are broken or missing. The fewer clips you have, the more difficult it will be to neatly roll the flaps up.
Now, to roll up the flap, stand on the mounting block inside the tent, almost right under the rope. Un-clip the clip that is closest to you (it is easier to clip onto the rope than it is to unclip. To unclip, you will need to push on the little finger and jiggle it away from the rope. To clip it back on, all you need to do is push the finger side against the rope).
Next, start to roll the flap up towards you, starting from the bottom and keeping that clip at the top accessible and centered. This might be difficult if the flap is new and thick or if you have a lot of clips fastened to the rope along the top. If you're having trouble, un-clip the neighbouring clips, too. Your flaps won't be rolled as tightly, but it will still work.
Once you have it rolled all the way to the top, flip the roll around 360 degrees and clip it back onto the rope. This will keep it rolled up!
Now move one or two fastened clips down the flap and repeat. This method works because you are alternating twisting one way (the original) and then the other (what we're doing here), so make sure that you always leave a clip or two between your rolls (otherwise when you flip the next one, it will just rotate a section of the flap and lose the tight fold).
To bring the flaps back down, just go along the flap and un-clip those twisted sections. They will unroll themselves and you can then clip back on to the rope or just leave those clips hanging if it doesn't sag too much.