There are two ways to travel from Kathmandu,Nepal to Lhasa of Tibet. By driving or by air. Since the earthquake in Nepal and Tibet area in 2014, the road from Kathmandu to Lhasa in the border area still has not been repaired. There is a way for the local people traveling through the border from Nepal to Tibet, but not for foreign travelers. So as for today, the road from Kathmandu to Lhasa through Zhangmu still closed. It looks like the road will be closed for a while.
Now the only way to travel from Kathmandu to Lhasa is by air. Before you take a flight from Kathmandu to Lhasa, you need to make sure that you have gotten your group Tibet visa with China visa. You also need to book a basic Tibet tour with a travel agency so that you will have no problems traveling in Tibet. If you do this way, no matter you have China visa before, you have to apply group China visa again, and plus a Tibet permit. All these have to be done while you are in Kathmandu.
So the best way for you is first travel to Tibet, and then fly from Lhasa to Kathmandu.