Any taxpayer or an unregistered person aggrieved by any decision or order passed against him by an adjudicating authority, may appeal to the Appellate Authority, within three months from the date on which the said decision or order is communicated to such person.

Order must be passed by the adjudicating authority for the taxpayer or an unregistered person to file an appeal to the Appellate Authority.

Navigate to  Services > User Services > My Applications > Application Type as Appeal to Appellate Authority > NEW APPLICATION button. Select the Order Type as Demand Order from the drop-down list.

The appellate authority may condone delay for a period of maximum of 1 month, if he is satisfied that the taxpayer was prevented by sufficient cause from presenting the appeal within the aforesaid period of three months and allow it to be presented within a period of one month after the expiry date of filing appeal.

Once an appeal against a demand order is filed, an email is sent to the taxpayer (or an unregistered person, as the case may be) and Appellate Authority. SMS is also sent to Taxpayer.
 
If the order is uploaded, Final Acknowledgement (APL-02) is issued by the Appellate Authority and the provisional acknowledgment date equals the filing date. 
 
If the order is not uploaded, a self-certified copy must be submitted within 7 days of filing APL-01. If submitted after 7 days, the submission date will be treated as the date of filing the appeal. The appeal is considered filed only when the final acknowledgment (APL-02) with an appeal number is issued. 
 
An appeal can be re-filed if rejected at the admission stage

Minimum of 10% of the disputed tax needs to be paid as pre-deposit (as per law) before filing an appeal. Lower percentage may be declared after approval from the competent authorities.

You can file the appeal either through DSC or EVC. DSC is mandatory for companies and LLPs.

Yes, if Appeal filed is admitted, the GST Portal flags the balance disputed amount as non-recoverable.