This is something that I see asked often and there is some simple coding that can be done to your theme to add notes or comments to the checkout, but we all know I love a work around (and I also don't code!). I use the Infinite Product Options app because it's easy to configure and customise...and it's free!
If you click on Apps (in your Shopify menu on the left) and then click on Visit the Shopify App Store you can search for "Infinite Product Options" and then click Infinite Product Options:
In the Infinite Product Options screen click on Add app - this will add the app to your Apps screen in Shopify. In the Apps screen of your store click on Infinite Product Options and then on the top right click on Installation > Installation and then choose Start Now under Automatic Installation.
Choose your current, published theme from the dropdown and then click Install
Back on the Dashboard the first thing you need to do is add the "Options" - these are the fields you want to appear on your product page (you will choose the product pages the field/s appear on in the next step). Click on Add Options:
Click on Add New (top left) and choose your "Display name" (this is the text that will appear before the field on the product screen), add the "Unique name" (this is what you will see when you assign the field to the product), you can then choose to add a "Tooltip" (extra info) or "Helptext".
You choose the type of field you want to display from the dropdown menu under "Type":
For this example I am going to add a gift box upgrade option for an additional $5:
I then hit Save and this field now appears in my Options list in the Options screen. To assign this field to a product (or products), go to Option Sets and click Add New.
The Create option set window will appear and you can choose to add as many Options from your newly created options list to each Option Set and then assign the Option Set to the products (I know, it's a bit confusing at first!).
You need to click on "Use option" beside each option as you choose it from the dropdown list (another trick it took me a while to learn):
Then you choose whether to assign the field/s to all of the products in your store or just some of the products in your store. To assign to certain products tick the Select custom products box. You can choose to filter by product vendor, type or tag or you can just click on Select Products and search for the products and then tick all those that apply:
Once you've added the products hit Save and the fields that you have designed and added to the "Option set" will then appear on the chosen products.
At any time you can add new fields (Options) and include them in an "Option set" and add them to the chosen products, and you can add different fields to a different "Option set" and then assign that field or group of fields to a different range of products.
It can take a bit of time to set up (especially if you want to add more than one field to a set and you want to use different sets for different products) but I find that it's effective and does the trick - even on a paid plan!
What I like about using an app (especially a free one) over coding is that you can change your theme and not worry about having to get it re-coded, there is also very minimal chance that the coding will glitch and cause an issue elsewhere (it does happen - often!).
If you haven't already make sure you join the Running In Heels Facebook Group where you can ask questions like this and any others you may have. If you need a hand installing the Infinite Product Option app please get in touch.