I downloaded 'letterland' apps, as they were a big part of my childhood learning.
I wasn't expecting DD to use them/be interested, but she was and has picked up the alphabet in a matter of days.
We also use Alphablocks (she is learning some spelling watching old episodes on youtube, again, I wasn't expecting this)
Have also had a go at Jolly Phonics, and DD can again sing all the songs with ease.
We've also had foam letters in the bath since she was small.
She can do her entire alphabet in name, sounds and letterland character name, she can also sing the right jolly phonics song for each letter if she feels like it.
Reading all that back I make it sound like DD is genius or I'm superior.
It just has been that easy, and I think it's because DD did it all in her own way, in her own time.
She LOVES the stories on letterland (There is a story, 2 or 3 songs for each letter, it teaches you to 'write' the letter and some games)
Before the LL apps I'd tried flashcards and some 'baby' alphabet books and just got bored looks!
As other PP have said, start with sounds. I'd taught my twin boys the alphabet 'properly' only to be told I'd taught them all wrong and they needed the sounds instead, they had speech delay, and got on just fine with the alphabet as it was, then got all confused going by sounds and it got very frustrating for them.