Hello,
Customer can add comments in the first step of the shopping procedure "Order and delivery comments". This is the only place where customer can add it.
There is not the option to add comments by customer in the products.
If you have some fixed options you can create variations.
For example; If you have "name cup".
You can create the product "cup" and variations with the most popular names.
You can see how many variations you can have per product in->Settings->General Settings->Utilisation
If your products are so personal that this option does not help you, you need to add a comment in every product for informing the customer that they should give the details in the shopping procedure or send you an e-mail.
Kind Regards,