Most of us make our own leaders. It gives us more leeway to adjust things away from the "norm" to fit specific conditions. I constantly adjust my leaders, take out a section here, add a section there, etc.
But for a beginner, there's nothing wrong with buying em till you get the basics down and understand what you're trying to do with a leader.
I generally follow the rule of 3rds. Generally, a third of the leader should be fairly flat, thick butt section. The middle third a more aggressive taper, and the final third is a fairly flat tippet. A 9' leader is about typical for your medium sized streams throwin normal sized nymphs and dries. Yes that includes tippet, so 3 ft butt, 3 ft taper, 3 ft tippet. So if you buy a 9 ft 5x leader, it should have everything already there, and the tippet is 5x. Tie it on and use it without adding anything, yet.
Now you've fished a while, changed your fly several times, broke one off while nymphing, etc. And you notice your tippet has shortened, right? Well, replace it. If you're just adding more 5x, it doesn't much matter whether you clip back first or not, just remember you want about 3 ft of tippet. If that end is frayed up, yeah, clip it back first. If it looks good, just add how much you lost.
As HA said, where you have to clip back is if you want to add heavier tippet. Say you're gonna use a big honkin drake, 5x is too fine a tippet to turn over big flies like that. You're gonna want 3x or 4x. But you don't want to add that after the 5x that's already there, that'd be going from thin to thick. It'll ruin the turnover, and if you break the line, you're gonna lose a lot of it! So you clip the leader back, at least as far as the diameter of tippet your going to add. Then you add the new, larger tippet.
And as was said, you should be adjusting tippet. Big flies like drakes, you want 3x or 4x on the end. Small flies like tricos and midges, you probably want 6x. If you're getting great drifts but can't hold accuracy and it's flopping everywhere, shorten the tippet. If you're being plenty accurate but struggling with drag, lengthen the tippet.
These adjustments are what the sport is all about. I use like 5-6 ft leaders, including tippet, on tight, thick brookie streams as well as when throwin streamers and buggers. Then when I'm fishing tricos on a smooth surfaced spring creek, I might have a 12 ft leader on! But if I had to pick a single length and stick with it for all situations, it'd probably be a 9 footer.
