Rhythm first, because it is half of it and considerably easier
Start with rhythm
Clap written rhythms without any pitch at all. A fortnight of ten minutes a day covers most of what you will ever meet.
Then pitch, by interval
Learn to read the distance between notes rather than naming each one. Reading by shape is how experienced players actually do it.
What not to do
Memorising mnemonics for line and space names. It produces slow letter-by-letter reading that is hard to unlearn later.
