Onesync can simply be enabled by adding to your server.cfg or executing in console
onesync_enabled 1
However some things/scripts might break, check out this topic [How-to] Adapt your script to OneSync and fix your issues
Info about onesync: The OneSync EAP and you
Info about onesync (new): OneSync+ and future work on OneSync
Onesync is completely free, you would only need Patreon perk if you wanted more slots.