There are different ways to create a singleton/shared instance of a class in Objective-C. Here I have explained two most preferred methods for it and both are thread safe.
1) Using "@synthesize"

    #define SINGLETON_FOR_CLASS(yourclassname) \
     \
    static yourclassname *shared##yourclassname = nil; \
     \
    + (yourclassname *)shared##yourclassname \
    { \
        @synchronized(self) \
        { \
            if (shared##yourclassname == nil) \
            { \
                shared##yourclassname = [[self alloc] init]; \
            } \
        } \
         \
        return shared##yourclassname; \
    } \
     \

2 ) Using "dispatch_once" from GCD

   #define SINGLETON_FOR_CLASS(yourclassname)\
            + (id) shared##yourclassname {\
            static dispatch_once_t pred = 0;\
            __strong static id _sharedObject = nil;\
            dispatch_once(&pred, ^{\
            _sharedObject = [[self alloc] init];\
            });\
            return _sharedObject;\
            }

Both the methods above are thread safe. There is only one difference between these two, dispatch_once is much faster than @synthesize.
I have created these methods as a macro. To use any of the method import the macro file in your class for which you need to create a shared instance.
Put following line in the header file of your class.
+(id)sharedyourclassname;

And in implementation class add following to create a singleton.

SINGLETON_FOR_CLASS(yourclassname);

Whatever class name you pass here it will create a singleton for that class.