Vote by mail (VBM) packets are mailed to all eligible voters by your County Elections Office.
Questions about your ballot (where to get a ballot, how to get a replacement ballot, where to drop off your ballot, what to do if you forget to sign your ballot, etc...) are best answered by your County Elections Office:
For more general information regarding the vote by mail process, read the Secretary of State's Vote By Mail Guide. (pdf format)
and Voting in Oregon Pamphlet (both documents are in pdf format). In addition, Multnomah County Elections has educational resources for the vote by mail process.