For modeling a CMMN, BPMN or DMN model there is the Flowable Cloud Design tool for which you can signup for free.
Go to https://www.flowable.com/account/open-source and signup for a new account. You will receive an email with a link to setup a password and then you will be able to sign-in to the Flowable Cloud landing page. From there you can go to the Flowable Cloud Design application and start modeling. You can also use the following tutorial link to get more information on how to get started with Flowable Cloud Design https://documentation.flowable.com/latest/howto/tutorial/how-put-together.