Add the Custom Fields plugin first
From the Project page > select the Trac service > Check the boxes of the plugins you need > Save Settings.
See here for more details on how to add plugins
Now add the Custom Fields on Trac
On your trac instance > Click Admin > Custom Fields > Fill form > Add
Please remove Custom Fields that have been created manually.
Some other Plugins, like the Time and Estimation plugin, will place Custom Fields automatically. To remove these Custom Fields entered by the plugins, please remove the plugins first. Failure to do so by just manually removing the Custom Field part of other plugin will break your instance. In that case, please enter a support case.