After receiving the appeal, the VA will mail the claimant a Statement of the Case describing what facts, laws and regulations were used in deciding the case. A VA Form 9 (Appeal to the Board of Veterans Appeals) will be included with the Statement of the Case.
If you wish to continue your appeal, you must complete and submit the VA Form 9 within 60 days of the mailing of the Statement of the Case, or within one year from the date the VA mailed its decision, whichever is later. Send your VA Form 9 to the local VA office handling your case; the office will file this and all related information in a claims folder, and will eventually forward it to the Board of Veterans Appeals for review.
On VA Form 9, make sure you clearly state the benefit you want and point out any mistakes you think the VA has made in its decision. If you submit new information or evidence with your VA Form 9, your local VA office, and it will prepare a Supplemental Statement of the Case (SSOC). If you are not satisfied with the SSOC, you have 60 days from the date the SSOC was mailed to submit, in writing, what you disagree with.
Usually, the above is all the paperwork you will need to send to your VA office. The one major exception is if you receive a SSOC covering a new issue. In this case, you must complete a supplemental VA Form 9 covering the new issue if you want to appeal it. For example, if you are appealing a pension ruling, and before the pension ruling is resolved, you appeal a medical payment ruling and receive an SSOC on the medical payment decision, you must send your VA office a VA Form 9 on the medical payment appeal.
It is possible to get an extension for the 60-day window you have to submit your VA Form 9 or respond to the SSOC. Simply write to your local VA office handling your appeal and explain why you need extra time to file.