Go to https://www.facebook.com/ in your browser. This will open your News Feed if you're logged into Facebook. If you aren't logged into Facebook, enter your email address and password to do so. Click your name in the top-right side of the Facebook page. You'll find this below the cover photo that's at the top of your Facebook page. Click a category tab (e.g., Your Photos) near the top of the page. Click a photo that you want to make private. This will open the photo. The photo must be one that you uploaded, not just one of you that someone else uploaded. This icon generally resembles a silhouette of a person (or two people) that you'll find below and to the right of your name in the upper-right side of the photo. A drop-down menu will appear. If clicking this icon results in a menu that says Edit Post Privacy, click Edit Post Privacy to go to the post, then click the privacy icon at the top of the post before continuing. It's in the drop-down menu. This option is in the expanded drop-down menu. Doing so will immediately change your photo's privacy so that only you can see it.

TL;DR in Vietnamese:
Mở Facebook. Đi đến trang cá nhân. Nhấp vào thẻ Photos (Hình ảnh). Chọn danh mục ảnh. Chọn ảnh. Nhấp vào biểu tượng "Privacy". Nhấp vào More… (Tùy chọn khác). Nhấp vào Only Me (Chỉ mình tôi).