Please take all measurements carefully, ideally using some help.

Measurements 1,2,3 and 4 are circumferences. 
While measuring do not hold measuring tape too tight or too loose. It should be neutral.

1) Forehead - measure just over your eyes

2) Mouth level - measure just under your mouth (between bottom lip and chin). Try to make the measuring tape lay naturally on the back of your neck in the shortest path. Please keep your mouth closed while measuring and head pointed straight forward.

3) Neck - measure in the middle of the neck.

4) Chin - measure from the tip of your chin to the back-top of your head.

5) Nose length - measure from the middle of the eyes level to the tip of your nose.

6) Nose base - measure from the point your nose connects with your face to the tip of your nose.

Sizes reference chart:

XS
S
M
L
XL
Forehead
52-53
54-55
56-57
58-59
60-61
Mouth level
38-40
41-44
45-48
49-51
52-54
Neck
28-31
32-34
35-38
39-42
43-46
*All sizes are in cm (1cm=0.3937inch)