Create signing.properties file either in your project root directory (next to your build.gradle), or globally in your gradle home directory ($HOME/.gradle).

Don't add it to your version control if you decided to keep it inside the project.

storeFile=keystore_file_path
storePAssword=keystore_password
keyAlias=key_alias
keyPassword=key_password

Add the following line to the end of your build.gradle:

apply from: 'https://raw.github.com/trikita/gradle-android-signing/master/gradle-android-signing.gradle'

gradle clean build

If your only want to build the release variant and sign it (without linting etc) - run gradle assembleRelease.