SFDCWallah

The answer is by using the “Custom settings methods ”.
Custom settings methods are all instance methods, that is, they are called by and operate on a particular instance of a custom setting.

In our Example, we have an object with API name as “Discount__c” and field name as “Percentage_Allowed__c”.

To get the org default value in Apex:
Discount__c d =  Discount__c.getOrgDefaults();
decimal discountValue= d.Percentage_Allowed__c;
system.debug(‘value is’ +discountValue);

To get the Specified profile  value in Apex :
string pid=’0056F000008ujg9QAA’;
Discount__c d =Discount__c.getInstance(pid);
decimal discountValue= d.Percentage_Allowed__c;
system.debug(‘value is ‘+discountValue);

To get the current profile’s value in Apex :
Discount__c d =Discount__c.getInstance(UserInfo.getProfileId());
decimal discountValue= d.Percentage_Allowed__c;
system.debug(‘value is’ +discountValue);

To get the User specified value in Apex :
string pid=’00e6F000002zYfJQAU’;
Discount__c d =  Discount__c.getInstance(pid);
decimal discountValue= d.Percentage_Allowed__c;
system.debug(‘value is’ +discountValue);

To get the  Current User’s value in Apex :
Discount__c d =  Discount__c.getInstance(UserInfo.getUserId());
getInstance(UserInfo.getProfileId());
decimal discountValue= d.Percentage_Allowed__c;
system.debug(‘value is’ +discountValue);