Use Tomcat Run/Debug Configuration. Click the 
button and choose new Remote configuration. In the Configuration tab copy the JVM options suggested by IDEA.
Run Tomcat with the suggested JVM options:
set JAVA_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,address=1317,suspend=n,server=y
startup
Press Debug button in IDEA.
Otherwise it’s the same as for Local Tomcat, check the tutorial.
Note that deploying to Tomcat is available only in IDEA Ultimate, but you can still manually deploy to Tomcat on your own and use Java’s remote debug features using the free edition.