Any new technique can take a while to learn, and waaaaaaaayyyyyy longer to master. To play slap proficiently (I'm still no master) I find that I have to relax.
Don't push it, just keep practicing and one day you'll have a "wow, that sounded pretty darn good" moment, so have a really wide grin at the ready.
Also, as a beginner I would heartily recommend taking it slow and concentrating on getting the notes clean (2nd most important thing in slap playing) and IN TIME (single most important thing when playing slap bass) - so I'd go for a metronome every time.
Slap bass technique can sometimes get a bit of a flaming on here, but if you're excited by the technique, and you really like the sound then go for it - don't feel that you shouldn't learn to slap, or that you are less of a 'musician' for slapping, because some internet people that you've never met tell you it's wrong.