That is an excellent page and where many of us started. On the other hand, I'm going to push my own page, which covers some things that I felt it overlooked. (Though you'll see that I refer to it frequently).
http://home.roadrunner.com/~computertaijutsu/ldap.html
Note what my page says about the migration scripts--they should make it easy to migrate users, but there are a bunch of other things that would be pulled along which you might not want.
Also, I'm not sure if the migration scripts include the objectclass of inetorgperson, which would be necessary for email.
If Slack doesn't have the scripts included in anything, they're easy enough to find.
http://www.padl.com/OSS/MigrationTools.html