Setting Issue Security Scheme on the Issue

Go to JIRA Administration > Issues > Issue Security Scheme

Create an Issue Security Scheme

Once created, click in "Security Levels"

Create as many levels as required

Go back to the Issue Security Schemes page

Click in Edit for the issue security scheme you just create levels for.

Make sure that the field "Default Security Level" is set to "None".

Go to the project you want to use this issue security scheme on.

Click in Project Settings

Scroll down to Issue Security

Click in the button "Actions" and then in "select issue security scheme."

Look for the issue security scheme you just created and click "Next"

Associate the scheme to the project

Once it's associated, you will have the levels of the issue security scheme there.

In the levels of the issue security, click in "Actions" and then in "Edit Issue Security Scheme"

In front of the level you want to edit, click in "Add"

Select the group/project role/user(etc) you want to be able to see the issues with that level

Make sure you have the Issue Security set on the issue Screen (Project Settings > Screens)

Go to the issue you want to set the issue security level to.

Scroll down to "Issue Security"

Choose the level you want for that issue and click "Update"

As Issue security schemes are provided by Jira, please take also a look at Configuring issue-level security  for Data Center or Configure issue security schemes  for the Cloud version.