This guide assumes you have Arma 3 installed, you can not do any terrain editing or use any of the Arma 3 Tools without having Arma 3 installed.
Once you made sure you have Arma 3 installed, you can install the Arma 3 Tools. Make sure you have the tools enabled in your Steam library, by clicking the dropdown menu above your games list.
Once Tools are enabled, search for Arma 3 Tools and install.
After installation click Launch, then read the license and accept and launch Terrain Builder. After launching you can close both Terrain Builder and Arma 3 Tools
When Arma Tools get minimized the most reliable way to reopen it, is right click the icon in the Task bar and click Show