The exact steps varies from phone model, country and some service providers also try to dictate terms as well… the up side is all Windows 8.1 phone have the basic same ability and you should have allowed the phone to run its automatic setup when you first got it home so the basic steps are;
1. Goto the phones ‘settings’,
2. Find and turn on the ‘network sharing’,
There are always two things in here: a broadcast name (bcn) and a password… the bcn is what name will show up in devices network settings so make sure you change this into something that makes sense to you. For example, if your phone is a nokia the bcn will be something along the lines of nokia_luma_ (random gibberish numbers) and that’s fine at home but when you go to the city there will be a hundred other people with mobiles.
The password is a good idea and just stops people connecting to your phone without asking and will again be some random numbers by default… always change it to something else but don’t use your name or the name of your kids/ pets.