If your issue/confusion can be summarised in only a few lines, and if the author of the original answer is still active, then it is often better to first leave a comment on the relevant answer and wait for the author to reply. The author may be able to answer your question, and may even choose to edit their answer as a result. (Thanks to Michael Albanese for pointing out in the comments that this should be your first step.)
If the author decides not to reply, then you should do both of the things suggested in the OP:
- Create a new question which links to the old one, and
- Put a link to the new question in a comment to the old.
Ideally when you write your new question, the reference to the old question should clear and useful, and not superficial. Doing this provides excellent context, and gives a starting point for someone trying to answer the question.
For example, in your specific instance you should say what you wrote here: "I am not arriving at the same result and I cannot find out where my reasoning fails", and then post your (failed) reasoning. Avoid dismissive references, such as: "My question is similar to this old question, but I didn't understand the answers there".