Redirects can be created by adding a new page with the page type "Redirect". The redirect may be to a page on your site, or on another domain.
For example, if you'd like www.yourdomain.com/old to redirect to your old domain, create a new Page called "Old" and put in www.olddomain.com as the Redirect. You may want to hide this page under a "Hidden Parent" page, with the visibility set to admins only, so that it doesn't clutter your site navigation.
You can bulk import redirects in Admin > Settings > Import > Redirects.