How write in CV about lecturing experience while being a PhD student

If lecturer is an actual title in your location, as it is in many, and you don't actually hold the title, then it would be a mistake to suggest otherwise. If you have full responsibility for a course then you could provide a list of "courses taught". But if you are just a guest lecturer in a course taught by another, you could provide a list of "presentations" with a bit of detail.

Don't say things that others might consider misleading.

You could also seek advice from your current department/school as to how to properly present yourself. Likewise, a professor that you worked for in such courses can provide some guidance.