Get Minecraft from the appstore and open the app.
Click the signin button, this will jump to an Xbox login screen.

Xbox login screen

If you don’t have a login, you will need to sign up for one. If you are logging in for your child then check out an overview of the process here.

Then do the following:

Once it has been saved then you can connect anytime you need. 

One thing to note is that by default Minecraft on the mobile and tablet needs wifi, there is a way to change that in the settings.

If the account is a child’s one then you will likely get this message:

You cannot play online multiplayer because of how your Microsoft Account is set up. Review and change your privacy settings at http://aka.ms/accountsettings

http://aka.ms/accountsettings – I have done a quick video guide here how to change the privacy settings, you will probably find that after you have done this then you need to log out of the MS account and re-login.