By far the best approach to Lewellyn is via Cwm Llafar from Bethesda.
This is a walk only to be attempted in fine weather,because Carnedd Lewellyn is one of the most challenging summits to navigate in the mist.
It's only 80 ft lower than Snowdon,so this is one big mountain, the third highest outside of Scotland, it deserves respect.
The easiest way up is via the Fynnon Llygwy Reservoir road, which is on the A5 around 4 miles from Ogwen Cottage.
An approach from Ogwen Cottage is a good one,but its a long walk and so dependant on good weather, navigation from Carnedd Dafydd to Lewellyn is not easy,and its very exposed to bad weather.
Here's my top three favourite ways up Lewellyn
1. Cwm Eigiau - from either the Craig yr Ysfa quarry route or the longer far more enjoyable route,which is possibly the easiest way up the mountain.
2. The Cwm Llafar Horseshoe, a walk with few equals in Northern Snowdonia.
3. Lewellyn via Yr Elen, parking your car in Gerlan.
Ogwen cottage is ok, but to really experience the magnificence of the third highest mountain out of Scotland, a less busy route is advisable.
That approach via Yr Elen is a gorgeous alternative, and its always very quiet.
Which ever route you eventually go for, make sure the weather forecast is good,because for someone totally new to the Carneddau navigation in poor weather is possibly more challenging in the Carneddau than any other area of Snowdonia.
I envy you, your first trip up Lewellyn, I wish I could turn the clock back and experience one of my fauvourite mountains for the first time.
Good Luck and keep safe.