When you are on 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 Slider Gallery option here.
Next to slider gallery option, there is a link that says Upload/Insert, click on it.
It will open pop-up which manages images for this item. Simply upload your images and Gallery tab with number of images attached to item should appear automatically at the top of pop-up screen. You can change image order and properties within this tab if you want. Close the pop-up, then update 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.