Ok, so the questions that you are asking are really "how to use Joomla" and not RT template related. So I would suggest Lynda.com to learn how to use Joomla.
how does one add an already created Joomla article to a RocketTheme template
You don't, articles are added to menu items not templates.
How does one create a new menu item to point to that article
The same way you do it in any other Joomla site. Menu manager > New menu item > menu type "single article" > pick article > save
How does one add a Google Maps location to the RocketTheme template
It's best to use a third-party module from this (many available on Joomla extensions directory (JED) - then just add that new module in module manager and publish it to the module position where you want it to appear on your site.
How does one add a slider to the RocketTheme template
The docs tell you how to recreate the demo content (which presume that you have read Gantry 5 docs and understand the basics of Gantry 5)
. Specifically slider docs are here
. Gantry 5 templates use "particles" - "slider" is one type of particle. Read the Gantry 5 docs to learn how to add a particle to the layout of your outline.
with RocketTheme it seems like having to learn a whole new language all over again...There is a learning curve to go through to learn Gantry 5 - the best method of understanding the basics is to read the Gantry 5 documentation and watch the videos therein and then load yourself a "rocketlauncher" package of the template. This gives you a faithful reproduction of the template demo all in one package so that you can see exactly how we did everything.
How does one add a video Player...
The Lexicon template includes a "video" particle - so you just drag that to the layout of your outline and fill in the particle details and Save the layout.
...that will show a catalog of videos
There is no particle in Lexicon or Gantry 5 core that will produce a catalog of videos. That said, you can copy particles (subject to your subscription type) from other templates and put that in Lexicon yourself. We have a particle called "videogrid" in other templates that would do that. If you want to know how to copy a Gantry 5 particle from one template to another please read my tutorial here in Gantry 5 Official Documentation
?.... Simple, straightforward videos that show all of those very essential and very basic items are what I am hoping to find....
There are simple and straightforward videos in the Gantry 5 docs. They will teach you the basics (for example) of how to use the layout manager to add particles to the layout of your outline. The Gantry 5 docs concentrate on the CORE Gantry 5 particles, but once you graps the concept then using any other (non-core template specific) particle is much the same. For the template specific particle documentation that is not shown in videos but rather the template documentation I've already linked you to.
So, I believe I have given you all the information I can on what resources are available to learn Gantry 5. We have hundreds of thousands of users who are using Gantry 5 successfully. Without doubt there is a learning curve to go through but, once learnt, the overwhelming response to Gantry 5 is that it is much more flexible, simpler, powerful, easier to customise, and faster than it's Gantry 4 predecessor.
I wish you luck with your project.
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