How can i display the jira.project.sidebar in my custom velocity template

Hi, welcome to the community.

Are you talking about something like adding this:

<meta name="decorator" content="atl.general"/>

if yes then also make sure include other tags you would need for a specific project I guess.

Check these threads/guides out please:
https://developer.atlassian.com/server/framework/atlassian-sdk/using-standard-page-decorators/