If you mean those from the marketplace, you just buy (click on the green buy button) then activate them onto your story (click on the change layout button at the bottom of the chapter/description before the comments section then click on activate). I explained it in more detail here:
https://www.asianfanfics.com/story/view/514589/11/visual-illusions
If you got codes from the coder, simply copy and paste the codes by clicking on 'Source' on the editor. Click on source again to see the layout. You can then change the text and images as how you normally do but if you want to change the colors for example, you'll have to go to the source code and change either the rgb codes (rgb (00,00,00)) or hex codes (#000000). The rgb and hex codes stands for a specific color.
If you're not into coding (or have no basic knowledge about it), I suggest using layouts from the marketplace since you don't have to worry about the codes. You just have to correctly place CONTENTID tags around your text to make them appear on the premade layout.
