The trail from Nefasit up to Debre Bizen is very obvious - you certainly dont need a guide to go up there. It is wide and well-trodden, lined with stones.
At the roadside near the trail-head in Nefasit there is a small store selling bottled mineral water. Get at least two bottles, as it is a long, and possibly quite hot walk up there.
Once on the way, you have beautiful views to reward you, getting better the further up you go. Keep an eye out for birds and wildlife such as rock hyraxes, too!
Well, the views alone make a visit here worthwhile. On a clear day, you could see all the way down to the Red Sea coast from here (I didn't). There are some great viewpoints from the monastery itself, but I also found it very interesting to follow the confusing, narrow trails outside to some of the higher hillocks outside.
These humble little houses scattered behind the main monastery buildings are the homes of the monks here. I felt it would be too nosey to poke around them too much.
Nefasit is about an hour from Asmara along the road to Massawa. With an early start, Debre Bizen can be visited as a stopover between those two cities.
It takes 2-3 hours to climb to the monastery from Nefasit.
Remember that you need to get a permit in Asmara before visiting!