When fitting your new halter it is important to note the following:

Adjust the halter to allow the noseband to lie above the cartilage, on the bone of the nose, 

this should be a snug (not tight) fit.  

The headband should also be adjusted behind the ears to ensure that there is never a chance of the noseband

slipping down.  Alpacas are nose breathers and will panic if the breathing is restricted.  

If you can insert more than one finger under the headband it is too slack. 

Never leave a haltered Alpaca unattended, if the halter gets caught up or the noseband slips 

this will cause the animal to panic.

Ensure that the Alpaca can still open its mouth to eat with the halter fitted.

Allow your Alpaca to become accustomed to the halter.  Little and often works best.