Installing a theme on SMF is actually much simpler than it sounds!
First thing, go into your admin control panel, find Configuration, then Themes and Layout. You can click the "Manage and Install" that comes up, or just click Themes and Layout, both go to the same place. Scroll to the bottom. You'll find a file upload field. Click it, navigate to your .zip or .tar.gz archive for the theme you want to install, select it, then click the Install! button to the bottom right. No, OneNote, I didn't tell you to capitalise that for a reason.
If all went well, it'll install itself. Enjoy!
First, f the theme you want to install does not install, you can upload it to your Themes folder in your file system (so if your forum is installed to /home/myawesomesite/public_html, your Themes folder is /home/myawesomesite/public_html/Themes). Upload the archive there, extract it, make sure it is in its own folder, and then return to the Manage and Install page, input the theme's folder name at the end of the "From a directory on the server:" input, then click install.
Do remember that the index.php and theme_info.xml files for the theme must be in the folder itself, not in a subfolder, so if you extract it and it doesn't put itself into a folder, you'll need to manually move all of the files and folders it just made into a folder within /Themes.
If it still does not work, contact the theme creator; they may have messed something up.
Second, you can create a copy of the default theme by inputting a theme name to the last input labelled "Create a copy of Default named:" - you can then customise this copy to your liking without needing to change the actual default Curve theme.