STM for Jira Cloud - Creating Bulk Issues
This page explains how to create bulk issues from within the issue page. Below is a video that takes you through creating bulk issues as well as written documentation.
The first step is to select to create bulk Epic issues, Linked issues, Sub-tasks or Unconnected issues. This document will go through creating bulk linked issues. The other pages are the same but they have less fields or different issue types.
When you select to create bulk issues the following page is displayed

Using an existing template
The easiest option from here is to create an existing template. To do this, select the template you want to create from the "Template To Create" dropdown at the top of the page. This will auto-fill the templates details into the template wizard below.

You can now select the issue type and link type to use (the link type isn't on the other create bulk pages).

If you don't want to make any changes then you can just click the Create Linked Issues button.
If you want to modify the template then you can navigate through the template using the Next and Prev buttons or by clicking on areas in the left-hand panel

See the Creating Custom Issues section for details on what the sections are and what can be changed.
Creating Custom Issues
This section deal with create issues that aren't defined by an existing template. It can also apply to modifying existing templates details. If an existing templates details are modified on this page then the changes are not saved back to the template. You need to use the STM Templates or Global STM Templates to modify a template.
To create custom issues you use a screen that is the same as the template editor.

For the details section, you don't need to enter a name but if you want yourself as a watcher on the issues then check the "Add current user as watcher" checkbox.
You can now go to the Create Issue section by clicking on the Next button or the Create Issues area of the left-hand panel. That will show details like that shown below.

You can now define the values of the issue you want to create. You can use the "Fields to display" dropdown to add or remove fields whose value you want to set. The rubbish bin in the top right can be used to remove this Create Issue section and the question marks on the left-hand side of the fields show information relevant to the field.
If you want to create another then you can click on the yellow plus next to the grey plus. This will have the same "Fields to display" as the last "Create Issue" created.

You can add as many of these issues as you wish, each with the fields that are required. When you have finished adding new issue details to create there are additional options that you can use. Click on the Path section of the left-hand panel and the panel below will be shown.

For details on the first two options see the STM documentation page. The Copy From Parent option allows you to copy the fields that belong to the parent of this issue. If you click on this option then the following page will be shown.

You can select as many fields as you want and those values will be copied into each issue created UNLESS that "Create Issue" sets a value for it.