Are you asking how to plan for DR or how to recover in a DR situation?
For recovery, you foillow your plan.
Creating the plan is a long process.
You can take classes on this.
Start with what do you do in a total disaster – ie: something (fire, tornado, etc) totally wipes out you primary place of business. IT Dept, Acct Dept, etc.
Now you need to determine how long can you be out of business before you are totally out of business. What is the ROI on repliacation of everything you need for business continuity. What is “everything you need”?
For IT, do you have a backup site, Data replication or do you rely on backup tapes?
For a full DR plan you need to go through every aspect of your business and determine what you would need to keep the businnes running in case of a DR.
The amount of time you could actually be down is proportionally the amount of investment needed to develop a safe/reliable DR plan.
You really need to look at some books on this as we could write comments here forever and not cover all you need to plan for.