In order to qualify for admission/readmission forgiveness, you must not have attended any accredited post-secondary institution during a period of years which the specific university in question specifies. You will also need to now earn good grades at the institution for a period of time; exactly how good the grades need to be and how long you have to earn them will again depend on the institution.
Once you believe you qualify for "forgiveness," you will need to petition the dean, writing a letter explaining and justifying your case. You will want to describe your motivation for returning to college, and why you believed you did not do well the first time and how that has changed.
Once the dean grants you forgiveness, your GPA will be recalculated without the inclusion of your old poor grades. These grades will still appear on your transcript, however, with the notation that they have been forgiven.
After earning forgiveness, this is a one-time occurence. Be sure to earn good grades from now on.