Guide? You mean 1,2,3 steps? or a general idea?
First, one could choose the restore path (i.e., 1.9.19+ no user/reset course backups to a 2.5, but all the files linked in the courses come in as 'legacy' - then one has to find a way to move those documents out of legacy into the new 2 file system.
One could also choose the site migrations ...*important* work with a copy of your current 1.9.19+ site ... the code directory and especially the data directory (it will undergo massive change) and the database (database also undergoes massive change). Working with a copy assures you've not lost it all on a botched migration.
First setp: 1.9.19+ to 2.2.highest and then I'd advise (if using git) to do the 'Moodle March' ... 2.2 -> 2.3 (*full* site backup/check functionality/convert stuff that needs converting like assignments, etc.._ -> 2.4 (*full* site backup) -> 2.5. Also think I'd pause at 2.5.
Check hosting situation for requirements of newer versions of Moodle - PHP version, MySQL version, do you have shared hosting? Size and other limitations might prove 'interesting'.
'spirit of sharing', Ken