Execute 'build.ps1' from your Custom Code working directory. This will package all files under the customcode directories and create a release that can be deployed via Octopus. Every time files are updated in any of the directories and 'build.ps1' is executed, a new release is created.
The following is an example of the output created during the build process where a version 0.3 of the Custom Code release is created. This release number shown after build completion will coincide with the version available in the Octopus interface

Figure 1 - Example of the output created during the build process
rebuild.ps1 forces a new release to be created regardless of whether there are new files or not.