Help for a newbie please - need to add text box to product page for personalisation

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Hi, I need to add a text box to a specific product page to enable customers to add personalised details for that product i.e. which scents they would like. I found a couple of online tutorials to add code but with my very limited understanding of coding they've lost me, mainly because of the complication that the theme I am using uses Sections. 

I know the code to use as I managed to get it to work but across all my products (which I don't want). The main issue is where to put that code. 

Thanks in advance


PS I can't use the variants option, because there are too many possible iterations of what the customer can choose.