It is very helpful to have all your Alpacas halter trained.
Even alpacas that may never leave your property may need to be separated from the herd or taken to the vet and it makes life so much easier if they can be led.
The book "The Camelid Companion - Handling and Training Your Alpacas & Llamas" by Marty McGee Bennett (ISBN 978-0970991607) is an excellent instruction book.
It is best not to be in a hurry or impatient about training your Alpacas. Five minutes a day, every other day, is enough to start with or your Alpaca will probably get fed up with you and just refuse to move.
After a few sessions in the shed they usually get used to the idea and we can progress to short walks. We usually take mothers with the young alpacas on their first walks outside the paddocks and then later on with another youngster until they are confident enough to go on their own.
Older alpacas who have come to your property and have never been halter trained can be treated the same way and usually learn much quicker we have found.
Having the right kind of halter is a must, ones that can be adjusted over the nose for a comfortable fit. We get ours from http://www.daisybankalpacas.com.au