Back when I used pure visual memo, I just went for it. The amount to memorize for a 3x3x3 is really not very much (even though it seems daunting until you try it), so you'll find you're probably able to handle it the very first time you try. But what you probably do want to do is to start by reviewing your memorization a whole bunch of times (maybe even 10 or 20) before you actually start solving. Since you're doing visual memorization, you can review an entire cube in just 10 to 20 seconds, if your memory is solid for it. And if your memory isn't solid, you needed another review.

When you're absolutely sure you remember it, then start solving. After a couple of successes this way, then start cutting back on the number of times you review; then you can gradually work your way down to not reviewing at all.
As for the actual memorization, I find it helpful to group several pieces together to memorize. 3 pieces at a time works, but for me, 4 is just as easy as 3, so I prefer 4. But you shouldn't need to go over the first 3 or 4 pieces a bunch of times before moving on - once or twice should be sufficient. I would recommend at first that you review your first type of piece (corners or edges, whichever you memorize first) quite a few times at first before you go on to the second type of piece.
Again, all this review is just to give you confidence and practice. Eventually, you'll want to cut out all that review. But when you're getting started, there's no reason not to review a bunch of times.