By following common sense, good design, and doing some research and reading about Google's suggestions to web developers.
You shouldn't have to hire someone to tell you what you can find out for free.
Also, you registered just to make this post?
I would suspect that a fair number of users have registered to make a post, so I'm not entirely certain what you're getting at. (Unless it's just curiosity, in which case I'll allow YorkSEO to speak for themselves).
Do companies hire you just to help build the number of links to their sites from other sites? How do you do this? Isn't that just plain old "marketing" at that point?
I'd label what you are referring to as more of online marketing to build a site/brand's presence and reputation, which is something that then influences a search engine ranking.
It shows up in hours on github because github has a lot of authority. It shows up high on google because I'm guessing you're searching for pretty long keyword strings. So if your ok with people finding your code when they search for "ruby gem for mysql connections" thats cool you don't need to hire an SEO professional. Hell you never need to hire an SEO professional. But if you want someone who knows more about it to help you with making sure Google and yahoo and bing can find your information then you can.
I really don't get the hatred on this site for SEO in general. I'm not an SEO professional myself but I know quite a few and now a days most people in the profession that are active and well known are all above board. They help you structure your information to make it easier to find and help you build relationships with other sources of traffic. Hardly evil stuff.
The goal of SEO, good SEO, is to build a site that caters to both users AND SEs. After all, users make you money, and ignoring one over the other is silly.
A classic example is AJAX. SEs can't pass it, and neither cana lot of users. A "good" designer may know this and buid a site to cater for these users, but how? Will it use forms (invisible to SEs) or an alternate structure?
Sure, a great web designer may "get" all this, but can you be sure? Given that SEs provide the largest, ongoing form of traffic online, why take the risk?
"I make something, like some small open-source software, put it on GitHub, and it shows up as the first result on Google within hours if you are searching specifically for something like it. I didn't have to hire a SEO."
Fallacy of dosage: ranking for a service with little competition is pretty easy. Capturing the whole raft of traffic available on a topic: difficult!!
Put down the coolaid.
Seriously -- Google is a business. They make money by selling the results of an algorithm. There's nothing magic or sacred about Google's algorithm. It makes mistakes. They blackball people. Even more critical, links are not a necessary and sufficient reason for an article to be the best thing I want to see. It's just a guessing game that works most of the time for most people.
If I had a cure for cancer that worked, and I put it on a website with a new domain tomorrow, and never SEO'd it, who do you think would come by?
Bupkis. Zero. Zilch. Nobody.
Good SEO is marketing. Marketing is a good thing. We need people to do things to bring to our attention stuff we might like. Google is not about to take the place of all marketing -- maybe that was a dream years ago but it's turned out to just be a fantasy on Google's part.
But if you also mentioned it to a few people (especially a doctor or cancer victim) then it would be featured in news articles, broadcast, tweeted etc. because it's a big deal. Soon your website would appear first in any relevant search.
That's the thing -- sure, perhaps all good ideas get great publicity and links simply because they're good ideas.
And perhaps all good business ideas make their owners lots of money simply because they're good ideas.
And perhaps all articles get referenced and linked based on their degree of usefulness to the world.
But in the real world, somebody has to go out and peddle ideas. That's marketing, sales, networking -- whatever you want to call it. Nowadays google is forcing us to go comment on other people's blogs, link-trade, and lord knows what else.
Good ideas simply don't always become popular simply because they're good and you tell a few people. It takes a lot more work than that.