I can’t vouch for third party components and have never personally used Skin Graft or MODx Resource Wizard. Those would rely on support from a 3rd party developer who is a bit busy being a new dad, and who is easily distracted by pirates. They’re not supported as any type of "official" way.
Installing a template really couldn’t be simpler:
1) Download a copy of the template, unzip it and read any instructions that come with it.
2) Upload your images/css to the webserver where the template instructions tell you to do so. If it’s not stated, open the template and the CSS files in your favorite text editor and read where the includes are set. Recall that MODx is intended to be set up by individuals who understand XHTML/CSS so this should be familiar territory.
3) Make sure you’ve got the right snippets and plugins installed in your site for the templates.
4) Make the chunks and templates in your manager under the Resources > Manage Resources tab. Copy and past the text from the example files provided by the template author.
5) Go to your documents and change the template, or use the Doc Manager module to change them all at once.
6) Use the site configuration to change the default template to your favorite new template.
Total time this should take: under 10 minutes, and probably closer to 5.
AGAIN, three critical guidelines to keep in mind:
[#] MODx is intended to be set up by individuals who are familiar with XHTML and CSS. You should at a bare minimum to be able to understand how to reference image, css and js paths in templates and in your css files.
[#] CSS/Images/JS typically reside on the filesystem and are referenced in pages that are generated from the DB. Make sure you use a base-href attribute in your template. If you don’t understand that, please refer to the first point.
[#] Templates and almost all add-ons are made by third-parties and the MODx project makes no claims that they’ll work as promised.
If you need assistance installing anything or if you get stuck, there’s also
a commercial support option.