How would you go about putting one of these: http://bootstraptour.com/
on you Joomla 3 site?
Should I make a module? Modify my template?
How would you do it?
I would not suggest editing your template as it's considered a core hack. Should you ever update your template to a newer version then you changes will be overridden. Instead I would suggest developing a plugin. This might however take some time, especially if you've never developed a plugin before. As an alternative, I've found 2 extensions for this exact feature:
Both are commercial (prices shown above). Unfortunately I did not find any free extensions on the JED site but one of the above might be worth the money if your under a tight deadline or do not feel confident enough with plugin development.
Hope this helps