Firstly, make sure all your optimized and resized images are in the right folder (single folder of their own).
Then go to File > Automate > Batch.
It will open a Dialog Box.
(1) Make sure your Action Set and watermark Action is set correctly. In the Set pop-up menu, select the set that contains the action you want to apply.
(2) In the Source pop-up menu, select Folder. Click the Choose button, navigate to the folder that you want to use, and click OK (in Windows) or Choose (in Mac OS).
Select other options in the Source area, as desired.
(3) In the Destination pop-up menu, select Folder where you want the processed images to go to. Click the Choose button, navigate to the folder that you want to use, and click OK (in Windows) or Choose (in Mac OS).
I usually choose the same folder to override the images without watermark. But you can have a different folder if you prefer.
(4) Specify how you want Photoshop to create the filenames for the new, processed files by selecting options from the drop-down menus.
When you process large numbers of files, these naming tools can help you keep track of when and how the files were created.
Tick and box you might think it’s useful to you.
In the Errors pop-up menu, select whether you want Photoshop to stop processing a batch when it encounters an error or whether you want it to simply continue and list the errors in a file. If you select the latter option, click the Save As button and, in the Save dialog box, specify a name and location for the log.
When you finish selecting options in the Batch dialog box, click OK to start the batch processing.
You will see flashes of lights in the screen that’s because it’s opening and closing images like crazy. It’s ok.
Once Photoshop stops. Go to your folder and check your photos. All of them should be watermarked by now. If not check your Action steps.
That’s it. Form now on and until you need to change something specific you don’t have to do anything else here. Just automate your watermark whenever your photos are ready to go and placed in your set folder.