SQL Server 2012 - login mapping to user
It is pretty simple, you just do the following two steps

1st: in sql server, create a new login using the user name in that database, like

use master;

create login [MyUserName] with password='mypassowrd123$', check_policy=off;
go


2nd: map the login with the user name inside that particular database, let's call it [xyz]

use [xyz]
alter user [MyUserName] with login = [MyUserName];
go