Itīs pretty complicated and quite frankly not worth it unless you have a pretty good job lined up with an operator.
There are 14 (fourteen, I kid you not) EASA theoretical tests required. Might be a few less if VFR only. Then you can start training for the CPL skill test.
There will be at least 4 checkrides before you can start working for an operator. You will need theoretical conversion course for the operator, CRM, Dangerous goods, first aid, fire fighting, ESE, Security & Heslo or part-SPO, part-M training for preflight and maintenance and possibly task training for maintenance related items depending on operator.
1. CPL skill test
2. Type rating skill test (You need type rating on each and every helicopter type in EASA).
3. Route/Role/Area & OPC conversion training and check (The operators checkride, probably similar to your part 135 checkride?).
4. Line training and line check
This is VFR single pilot only. There is more for multicrew & IFR.