How do I tune a Bandsaw so the blade doesn't drift?
The users manual for the saw should have all the information you need to get the saw setup. This is important because each model of saw will have different guidelines. If it is new saw such manual should have been in the box with the unit. 

For a used saw that you purchased at some auction, sale or craig's listing there was not likely a users manual delivered with the unit.  :^(

There is help available on line though. Do a search on "Delta Bandsaw Users Manual" will turn up a good bit of information.

One link I found was here. 

Key items that they list that would be applicable to band saws:


Ensure the blade is the correct length
Make sure the teeth are pointing in the right direction (up/down) when blade is installed.
Set the tension levers to apply tension to the blade
Hand rotate the tires that the blade rotates on to see if the blade tracks and centers.
If there problem then check the blade tension and adjust if necessary
Finally if the blade seems to want to come off the wheels it will be necessary adjust the coplanarity of the wheels to make sure that they are correct.