Recall that STS 4.5.8 (from hereon in, referred to as STS) comes with 4 modules (or sub-modules if you like). The Default module must be enabled to use STS at all.
This is the main module. Must be enabled to use any of the others.
Even if you just have only this one enabled, it will already make your designing job much easier.
In OSC Admin, you enable/disable it. There are few parameters you can set. For now, I am only dealing with the basic ones we need.

As I said earlier, we would be choosing the /test/ Template folder as our working folder. If you had another Template folder you wanted to use and you wanted the default Template-file to still be sts_template.html, then you would move (or copy) that file over to your Template folder of choice. The Working Template folder you choose is just the folder that contains all your HTML template files and all you have to do to choose/change the Template folder you want to use, is by letting STS know - via the osc's Admin > Modules; just as we are doing now.
I know the smarter ones amongst you will find this silly but it took me a quite awhile before I could see these connection clearly. So I am including a summary-logic here for those of you who might unenviably find themselves in a similar situation as myself.
What we have done here is:
OK! Great! But what do you do now, right? Well you can now simply go to your chosen default HTML template file:
/includes/sts_templates/test/sts_template.html
Open it in any HTML editor, and make changes to layout as you like.
And if you need to know either the definition or more placeholders you can use, please refer to Bill Kellum's STS v4.5.8 User Manual (which comes with the STS module when you download it). Or if you prefer, you can download this STS v4.5.8 User Manual (in .pdf format; approximately 273 kb. This is Bill Kellum's documentation but I just made it into .pdf format as it is safer for internet downloads).
When you open up the folder /sts_inc/ containing the placeholder files, you will see that there are a number of placeholder files in there already. If you wanted to use some placeholder that is in one of this other placeholder files (in addition to the default sts_user_code.php), then in your Default module setup, you just add the name of the additional placeholder file(s) after sts_user_code.php separated by semi-color (;).
For example, if you wanted to use a placeholder file you created and named extras.php, then your setup would look like:
Files
for normal template = sts_user_code.php;extras.php
You can also create your own placeholders and put it in a new file or in one of the existing placeholder files. Then remember to "tell" STS as per above. If you create a new placeholder file, you must put it in the same folder as with all the other placeholder files - i.e. in folder /sts_inc/