Let's assume that you wish to have a custom issue relating to meta descriptions. For illustration purposes, let's assume you prefer to create an issue with meta descriptions being >160 characters.
Within any project, under "Data explorer" report, you can easily create an issue based on the specific attributes and after you are done, click "Save issue as".
Then, create a name for your new custom issue, select the category type for this issue as well as the level of importance. You can also choose to add to new projects or for all projects for that matter. Once you are done, click "Create".
To view the issue created, head back to Global Settings or Issue Settings and right at the top, you will be able to select "Custom" to see the issues you have created.
Here, you can see the issue that you have created. You can select the level or importance and toggle it on or off.