Log into your hosting space via a FTP software (eg. FileZilla).
Upload the Stash theme folder (the unzipped one) into wp-content/themes in your WordPress installation.
Login into your admin section in WordPress (http://your-site.com/wp-admin)
Go to Appearance - Themes
Activate the installed theme.
Stash comes with some bundled plugins that need to be installed. After theme activation a notice message will require you to install those plugins. Select all, and click Install.
Our themes use Hybrid Composer plugin to run so make sure you install and activate it. Go to Plugins and activate Hybrid Composer. Don't worry, it's a plugin developed by us and it's completely integrated with the theme.