When you are on a portfolio item single page in admin, look at the bottom of the page for a section box named Media Display Options which has three options. Select the Slider Gallery option here.
Next to the slider gallery option, there is a link that says Upload/Insert, click on it.
It will open a pop-up that manages images for this item. Simply upload your images and a Gallery tab with the number of images attached to the item should appear automatically at the top of the pop-up screen. You can change the image order and properties within this tab if you want. Close the pop-up, then update the post, and that’s it!
Note: You can’t add already uploaded images attached to some other item (post) – you need to upload new images. That’s because WordPress itself doesn’t allow one specific image to be attached to multiple posts. So if you want the same image to appear for different posts, you need to upload it twice.