How can I open a Code Editor in Customise?

I am trying to add a code which will unable me to have a heading over my text with columns (as per my other question on my profile) however, I have no idea how to open the code editor in customise.

Hi,

Please refer this

Editing theme code · Shopify Help Center

Thanks

Hit Like and Accept as Solution.

Unfortunately, this does not show me how to open the code editor in the customise mode.

Hi,

Follow this if this can help you.

How to view and edit the shopify store theme code - liquid files - YouTube

Thanks