Well, your template
is the markup.
A typical workflow (for me) is to develop the page as if I was building a static page. Build out the html/css/javascrpt etc in your editor of choice.
I will use lorem ipsum to simulate content even. When I have my site looking the way I want, I replace my main content area of my mockup with this tag:
And my <h1> mockup with this tag:
These are 2 modx tags that will be used in nearly every template so I like to put them in right away so I can test my page.
Now go into modx, and click the "Elements" tab. From there, you will want to create a new template, and paste in all of the html that you have just built. Be sure to give your template a name, then save it.
Then click on the "Resources" tab, and create a new resource. You could also click on the existing resource - the one with the (1) next to it.
Let’s say you clicked on the existing resource. When you click on it, it will take you to the "edit resource" view.
From there choose your newly created template. Be sure to click "Save" on the upper right corner. Now the page you are looking at is using the template (the html) that you built. You will not see any of the markup in the "Resource Content" area. That area is for your content. By content I mean the words/pictures that you want on your site. Type anything in there, this is just a test page. You will also need to give your page a title.
Once you have all of that information click "Save" once again, and then click "Preview". You should the text that you wrote in the "Content Resource" field. If you view the page source, you will see that the page is using your template.