When a job is posted on Elance, the employer specifies if they prefer “open” or “sealed” proposals. 

If an employer selects “sealed” proposals, the proposal details and proposal amount will only be visible to the employer and the provider that submitted the proposal. You will be able to see your proposal, but other providers will not be able to see yours and vice versa.

As a provider, there is nothing that you need to do to seal your proposal. It is either sealed or not based on the employer’s preference when the job is first posted.