In reply to phja:
This old chesnut! Fortunately everyone seems to be giving the right advice, which is unusual as I've seen it described wrongly in books and online many times.
Just to reiterate what others have said here is my guide to finding the gullies.
Looking at Bristly from the decent of Tryfan's South Ridge (or from Bwlch Tryfan, but the view is less good there) you can see an obviously squarish dark gully a good few metres wide, well to the right of the wall. If you look closely ytou can also see a vague terrace cutting across Bristly quite low down (i.e. well before it becomes a ridge, perhaps 10m up). If you are looking at the gully I described it is the only one with an obvious continuation below this terrace, right down to the grass/scree. This obvious gully is Dexter. Many, many people go up this way as it is obvious and it looks 'sinister'. However judging from my own experience and that of others posting here, it has become dangerously unstable in the upper part (it was already dangerous enough because parties above frequently dislodge debris).
To find Sinister, visually follow the bottom of Dexter up to the terrace and then squint hard and you can see a small drystone wall just up and to the left (not the main wall which crosses the bwlch). Sinister starts just beyond (i.e. left of) this wall.
I've made a little illustration which I really hope is right or I'm going to feel like an idiot...
https://flic.kr/p/ugvxXp