Well, I don't know if there is really a best order to do these in, I'd way which ones are more important. I tend to put off the really big hacks until last just due to the constant revisions done on them.
In my opinion, I'd start with the easy ones first. Creating backups of the original files as you go and save the files in folders of their own for each hack, and labeling the order of which the hacks were installed. Like. 1st - Last reply , 2nd - PM Notification, etc... Just so if you have to fall back you know what order you did them in.
Here the order I'd do them in though.
- Last to reply to post
Stars/Custom Rank
PM Notify on Main Page
Slash UBB and Latest Topics
Who's Online
Calendar
Start off with the finished hacks first, then work your way down.