@Hòa Nghiêm Nguyễn Kudos for trying, but please read the first post of the link down below, in order to get an idea for what's required for cuts: 

https://sortitoutsi.net/content/360/guide-to-cutting-and-assorted-hints-tips-and-assistance

We ask for all cuts to include a collar. Here's a video on how to achieve this when the source image doesn't necessarily allow it:

I recommend using remove.bg for your cuts. It's a fantastic free online tool which does the heavy lifting for you by removing the background in the source image, leaving you to only rotate, crop and/or nudge the collar upwards. It's tremendously easy to use and provides fantastic quality. With it, you can go from source to cut in a couple of minutes for the vast majority of images.

Here's a couple of videos on how to use it, one when combined with Photoshop and another using photopea.com, which is a free online app so close to Photoshop that it has the same shortcuts, layout and icons 😉 If you don't have Photoshop, photopea.com is the tool I would recommend to use to tweak the draft cut produced by remove.bg. It's more powerful and versatile than GIMP and other similar tools.