Yes, the installation is very simple.
If any scripts have any non-standard installation, they normally explain that in the first post of the plugin thread.
The general idea is this. Single .rb files (like progressbar.rb), just get placed in the Sketchup\Plugins folder.
If it is a .zip file that contains lots of files and folders, then the author normally has it zipped in such a way that you can just open the zip file, select everything and drag it right into your plugins folder and they script will be installed correctly.
There are a couple of scripts that require you to install other scripts (some scripts are dependent on other scripts to work). In those cases, just install the other scripts just like I described above.
On top of this, you need to be aware that not all scripts get placed in the "plugins" menu. They can be put in the right click context menu, "plugins", "draw", "tools", "file", etc. They can install themselves into any main menu in SketchUp. So sometimes you have to hunt around for them.
If you have questions about specific scripts beyond these generaly questions, you can always ask in that plugin's thread.
Hope that begins to help,
Chris