Use ResEdit to put the 'LABL' template into your ResEdit Preferences file (insert standard warning about using a copy of the file instead of the original). (if you don't know what I mean you should learn how to use ResEdit before continuing.
Now from ResEdit you can create a new file with the Creator of 'SLpk' and a Type of 'SLIB'. The file needs 3 resource types 'PREC', 'LABL' & 'ICON'. The PREC ID #4 is the Page size for this label. The 'ICON' has to resources #5 & #6 they are the bottom and top half of the thumbnail picture of the label.
The 'LABL' resource is the description of the label itself. it must have an ID of #0.
field what it is
========== =====================
Size of window Rectangle of the size of the window to be displayed for this label.
Label White Rectangle of the actual size of the label. (in coords of the Label Frame)
Label Frame Rectangle from one label to the next (Includes borders & rims)
Label Used Rectangle of the space you would like to be able to use (Frame Coords)
RoundRect
Height Amount by which to round the corners of the label(IM-V1 Quickdraw)
RoundRect
Width Amount by which to round the corners of the label
# of Rows Number of rows on a page
# of Cols Number of columns on a page
Left Border Any space on the left side of the page that should not be used (tractor feed)
Top Border Any space on the top side of the page that should not be used.
It will probably be easer to copy a file that is included and Edit it to make a new one.
(BTW: all refs in the 'LABL' template use quickdraw coords & sizes. So, use 73=1 inch)
(Also, having said that the 'PREC' resource height & width are 120=1 in [don't ask me why])
Good luck & if you make labels Files please send them to me, so I can include them in future releases.