The are two commonly used starting points for this route. The description is for the eastern approach via Tryfan Bach (Little Tryfan), which is slightly less steep at first. An alternative start is up the boulder field below Milestone Buttress, which joins this route above 1/3 of the way up.
Starting from the road, head along the path to the farm buildings and campsite. Pass to the left of the farm house and head up the obvious and well surfaced path which goes towards the mountain, which looms above you.
Go up the steps and when you reach the fence line, turn right and start to climb steeply. Soon you'll emerge from a steep rocky gully onto a rocky area. This is something of a junction on Tryfan, as the Heather Terrace path starts from here, and the path from near the lake joins. If you feel uncomfortable with the exposure so far, now would be a good time to turn around or try the 'Heather Terrace' path, as things get a lot worse. You now need to go southwards, up the rocks. There is no one clear path, and you'll need to ensure you are on the ridge, as going to far to the left puts you on the Heather Terrace path, and going to far right puts you into very steep ground. Although nothing is really hard, you'll start to feel a sense of exposure, as the road and the lake looks a long way below you now.
After a climbing up steeply, you'll reach a cairn and a flatish area covered in quartz (white rock). It's worth making an effort to remember what you have passed, so if you need to retreat you can go back the same way. Enjoy the briefly flat ground, and walk along the top of the broad ridge. You should have a real sense of being high above the valley, with steep drops on 3 sides, and a massive wall of rock in front of you. Walk up the grassy slope in front of you, using the zig zagging path, until you reach a point where a large wall of bare rock blocks your way.
If you are confident and conditions are good, you can scramble up this to directly gain the north summit of Tryfan. A slightly easier route is to follow the path which goes left around the wall. The path drops a few metres, before carrying along below the actual rock face. Most of the path is easy flat walking. Interspersed with the flat easy bits, there are on or two places where some scrambling is required as you climb steep sections which to go around some of the butresses. These aren't particularly hard, but the mossy rock can be slippery in the wet, and the consequences of a fall extremely dire. At ground level few of the obstacles would cause much concern, you will the huge exposure to the left, you'll probably be concentrating very hard. After what is only a few hundred metres you will have one final short climb up either a mossy chocked crack, or a very exposed traverse. You'll then reach a rocky platform (I once bivvied here!), where you are just below the top of the ridge. In front of you (south) should see the summit. Descend from the rocky platform, and pass around the top of the grassy gully, before scrambling up the rocky notch on the right. You are now only a few metres from the summit across the boulder covered summit plateau. A leap between the two rocks called Adam and Eve offers dare-devils one more chance to show off. Once you have tired of the summit, which can be a cold and windy place even in summer, as so far you have been protected from the prevailing winds, it's time to return to ground level.
The safest way off the summit is to carry on along the south ridge. Repeating the north ridge is not recommended, as descending is harder and more dangerous, especially if you are tired from the ascent. From the summit follow the 'path', or rather the trail of footworn boulders which trends SE. Take care on the descent, as you need to pay attention to the direction you are heading, and it's easy to go wrong in poor visibility. Following the main south ridge is a much harder line. After decending the jumbled mass of boulders, the ground starts to become easier. Make sure you avoid the temptation to descend too far to the right, as you need to head for the col at Bwlch Tryfan. From the col cross the fence line and head down towards the obvious track. You'll probably find you need to head over rough grassy pickup the path below you. Follow this path along the base of the mountain, and thence to where you rejoin the path from the farm.