Tutorial??? Uhhhh ... more like suggestions/insight(?) ... guesses ... due to factors unknown. 
Stand up the 3.5.1 stock.
Then, research addon plugins that existed in all of the 2.6.'s. Compat versions in 3.5.x? Install compat versions of those plugins in the 3.5.1.
In one of the Mnet (will call it Mnet Node A), setup automated backups for content but no users - that's because users would be authenticating via MNet. Save the backups outside of moodledata to a designated directory where you can actually see the filenames and files. If linux, that could be a mount point on the 3.5.x setup. Make one run of automated backups on that server (then turn off automated backups on that server).
If no mountpoint for the 3.5.1 setup, xfer the backups to the 3.5.1 ... file system repository ... directory named from the Mnet Node courses were coming from /moodledata/repository/mneta/
Then in the 3.5.1 restore a course.
Test the course.
If all goes well, proceed with restoring the other backups from Mnet Node A.
All goes well?
Then do same for Mnet Node B.
Then Mnet Node C.
If that's a lot of stuff to do, hopefully you have Linux and could install moosh on the 3.5.1 server.
Via moosh (command line only) one can restore multiple backups to a category in short order via bash shell using a looping script to call moosh to restore a .mbz to a category.
This last suggestion I have done moving courses from an old 14.04 Ubuntu server ... older version of Moodle (but not by much) to a brand new 16.04 server with a higher version of Moodle. It did take a full day and re-write of the bash shell script to pickup backups that were stored in directories named for categories - sci directory had all the science backups ... restored to a previously created category in new server for sci courses.
'spirit of sharing', Ken
