Tap the Skype app icon, which resembles a blue "S" on the white Skype logo. If you haven't yet downloaded the Skype app, you can do so for free using the iPhone's App Store or the Android's Google Play Store. It's near the bottom of the screen. Doing so brings up the Skype If you're currently logged into a different Skype account, tap your profile image or the ☰ icon and tap Sign out before proceeding. Type your phone number into the text field in the middle of the screen.  If you'd rather use your email address, tap Use your email instead below the Back button, then enter your email address. You will eventually need to add your phone number before you can use Skype. It's a blue button near the bottom of the screen. In the "Create a password" text box, type in the password that you want to use for your Skype account.  Do so in the "First name" text field and the "Last name" text field, respectively.  Tap the "Month" drop-down box and select your month of birth, then repeat with the "Day" and "Year" drop-down boxes.  Depending on whether you signed up with a phone number or an email address, this step will vary:   Text - Open your phone's Messages app, open the text from Skype, and note the four-digit code in the message. Type that code into the "Enter code" text box.  Email - Open your email inbox, open the "Verify your email address" email from "Microsoft account team", and note the bold, four-digit code in the email. Type that code into the "Enter code" text box. Doing so will confirm your phone number or email address, thus creating your Skype account. Your Skype app's setup page will open after this point. If you didn't sign up with a phone number, you'll be prompted to add and verify a phone number before you can continue. You can tap Skip in the top-right corner of the screen until you arrive at the main Skype interface to begin using Skype immediately, or you can set up your Skype app by doing the following:  Select a theme (Light or Dark) Tap → twice. Allow Skype to access your contacts by tapping OK...

TL;DR in Vietnamese:
Mở Skype. Chọn Create Account (Tạo tài khoản) ở gần cuối màn hình để chuyển sang trang đăng ký Skype. Nhập số điện thoại. Chọn nút Next (Tiếp tục) màu xanh dương ở gần cuối màn hình. Nhập mật khẩu. Chọn Next (Tiếp tục). Nhập họ và tên của bạn. Chọn Next (Tiếp tục). Chọn ngày sinh của bạn. Chọn Next (Tiếp tục). Xác minh tài khoản. Chọn Next (Tiếp tục). Điều chỉnh thiết lập của ứng dụng Skype.