So I'm going to start with my favorite fabric mockup from Creatsy that I've been using for a long time and many times. Here is the link that you can also find it easier in Creative market.
Create a tile with 3000 x 3000 or even smaller by tapping Create new and I’m sure you’re familiar with this step then you drag your tile from your computer to Photoshop then you will see your pattern tile fit your document like in the photo.
(Your tile and document don’t need to be the same size. My tile was 5000x5000px but the document 3000x3000)
Tap enter to place your tile then go to step 2.
Go to Edit then Define pattern. This step will save your pattern and turn it into smart object if your file is PNG or JPEG format. It will ask you to name the pattern whatever you want.
Now go to Window -> Patterns to see your pattern on the panel
Now you got it here
Open your mockup file
Go to the Textures where the red rectangle is in the photo. Tap it and it will take you to another “room” or where you can apply your pattern to the mockup.
It looks like this:
Tap your pattern. This panel is always here or in every file you open until you switch it off.
And you get this
Scale your tile
If you want to scale your pattern just tap your pattern layer you can see it where the little hand is in the photo
This window will appear to allow you to scale your pattern.
Your pattern is in smart object so it will not loose its quality
When you get the right scale for you then time for closing and save the scale that you want to see on this fabric mockup.
Close the window by using shortcut Command + W
You can save your file from step 1- 3 to use later. Save as many patterns as you want there then every time you apply on mockup will be faster. But it also depends on your computer memory. Sometimes maybe Photoshop doesn’t let you save too many pattern because you don't have many spaces remaining in your computer.
Hope it can help you apply your pattern with Creatsy mockups as well!
Check out these favourites of mine: