How do I tie an issue to a checklist?

Web, via Checklists:

1. Navigate to a checklist item

2. Click " add issue"

3. Click Add new issue if you want to create a new issue tied to the checklist. 
Click on an existing issue to tie the issue to the checklist.

Web, via Issues:

1. Navigate to the Issue

2. Click the “+” button in the bottom left of the Discussion tab and select “Checklist item”

3. Click on the checklist item you want and hit “Select”

iPhone

1. Navigate to a checklist item 

2. Press on add next to related issues. 
Select Create new issue to create a new related issue
Select Add existing issue to tie an existing issue to this checklist. 

Android

1. Navigate to a checklist item 

2. Press on + next to related issues. 
Select Create new issue to create a new related issue
Select Add existing issue to tie an existing issue to this checklist.