After you finish CPT and pass the written, find a flight school or club with a light twin and get some instruction in it until you can consistently perform the ATP ACS tasks to standard. Unless things have changed in the last few years, a stand alone ATP (i.e. with no type) is self endorsed, so you can make arrangements with a DPE as soon as you’re ready…and be absolutely sure that you meet all of the minimums and requirements before you test.