Download meta trader file from brokers web site and install it on your pc.
After installed, automatically you will get the option to open an account, choose demo and open it.
If not get the automatic offer or like to open later, can follow the below steps.
Open meta trader > go to File > go to Open An Account > select Meta Quotes Demo > click Next > select New Demo Account > click Next > now fill up the Form > click I Agree... > click Next > click Finish.
Now completed. When you fill up the form, you can choose Demo USD, Leverage, Amount etc.
Registered demo account will be live in your terminal.
Hope you are very clear.
Thanks and best of luck.