After creating an unsplit Info file, you must create a tag table for it. Visit the Info file you wish to tagify and type:
M-x Info-tagify
(Note the upper case I in Info-tagify.) This creates an
Info file with a tag table that you can validate.
The third step is to validate the Info file:
M-x Info-validate
(Note the upper case I in Info-validate.)
In brief, the steps are:
C-u M-x texinfo-format-buffer M-x Info-tagify M-x Info-validate
After you have validated the node structure, you can rerun
texinfo-format-buffer in the normal way so it will construct a
tag table and split the file automatically, or you can make the tag
table and split the file manually.