How should I install GSIs using TWRP?

Thank you for your reply. After I saw it, I re-installed TWRP and new problem has emerged. The .img.xz file won’t appear. I googled it, and saw someone saying that I should extract the .img.xz file using 7zip to .img file. Is that correct?

+) I’ve extracted the .img.xz file to .img file and now it appears. But, still there are only two options-boot and recovery. There are not any system option. What should I do?

++) I’m not sure this information will be helpful, but what I’m trying to installing is AOSP