If you are going to install the theme using FTP, you will need an FTP Client, such as FileZilla..
When you download the theme from Themeforest, you will get an archive file in .zip extension.
You need to unzip that archive. When done, you should see WeBuild Folder (folder name can be changed from version to version).
Please, follow the steps below to install the theme via FTP:
Step 1 - Log into your hosting space (server) via FTP client.
Step 2 - In extracted archive folder, find WeBuild folder
Step 3 - Upload the WeBuild folder to your server in path .../wp-content/themes/.
Step 4 - The uploaded path should be like this: .../wp-content/themes/webuild/
Step 5 - Login to your WordPress Dashdoard and go to Appearance > Themes and activate the WeBuild Theme.
Step 6 - After activating the theme a notice should appear on the top of the screen, click on begin installing plugins ( see screenshots below ).
Step 7 - Select all the plugin, change Bulk Actions drop to Install then click apply ( see screenshots below ).
Step 8 - Congratulation you are ready to use We Build, check the First Steps part of our documentation to see what you may want to do before starting to edit your WordPress Site.