Step 1 : Open the editor and click Add block / Add Section. This opens a selection of blocks or apps that you can add.
Step 2 : Select the F: HTML/CSS : JS Liquid Code app. It will add a new block to your store.
Step 3 : Click on the newly added app block to open the code editor, where you can write your custom code. 
A new code block will be added to your store. You can easily change its position by dragging and dropping the block to your desired location. 
Video Tutorial Link : https://www.youtube.com/watch?v=we0ueLqPKdQ
If you encounter any issues, please let us know. We are happy to help!