Instillation of Lava theme is very easy. Its the same method that you follow for installation of other Drupal themes.
Step 1: Download the theme
Download the latest version of Lava theme from project page.
The download file will be a compressed .zip or .tar.gz file
Step 2: Extract the files
Right click the zip file and extract the zip file.
After extracting the zip file you will get a folder named lava
Step 3: Upload the theme folder
Upload this lava folder to sites/all/themes directory.
Step 4: Enable the theme
Now go to administer > site building > themes
Click Enable and set default link below the theme.
Step 5: Perform basic theme settings
After you have enabled the Lava theme, perform few basic settings from domain.com/admin/appearance/settings/lava or by clicking Settings link.