You know the "g *search term*" to search *search term* on Google, and same for Wikipedia etc.?
I use them all the time, and I have added a couple of my own, and decided to share a little tutorial on how to get started on making them yourself.
I will be using a different language Wikipedia (finnish as I am finnish) search as an example, as it is simple and gives you the idea of what we're doing here.
Step 1: Explore the Site You're Adding
In this case we're going to the finnish Wikipedia's main page. Then you need to think how you are going to use the search engine once we've set it up. Wikipedia is pretty simple, and there is really just one way to use it, and that is to read articles.
So go to any article on Wikipedia and copy the whole address on notepad or something.
Step 2: Examine the Address
What we need to do is recognise that what part of the address is "static", which means that whatever you search for it will be the same. In this case http://fi.wikipedia.org/wiki/
And the part that says what you searched for is "Opera" (in the wikipedia article of Opera, of course)
Step 3: Finally, Add the Search Engine
Now, you should choose a name for it that describes the search engine. For finnish Wikipedia, we shall use the name Wikipedia Finland. Clever, huh?
Choose a keyword. This is the 'g' in Google search, and 'w' in Wikipedia search. As it is Wikipedia Finland, I'll choose 'wf'.
Now here's the part where we make it work. The Address-field. Copy the static part of the address in it, in this case 'fi.wikipedia.org/wiki/'. Then where the search term is, put '%s'. As Opera is coded in C++, %s means a string variable, and here %s means the thing you searched for.
So simply if you search Finnish Wiki for Instructables article, which doesn't exist :(, you write 'wf instructables' in the address bar, and the %s means the keyword instructables. Get it?
Step 4: Other Examples
A hard one is Google's image search. It's hard because when you search for something, the address looks like this: https://www.google.com/images?q=something&biw=1920&bih=948&sei=dH8eUJ7FBamk4ATEs4C4DA
and that's pretty long. I have no idea what 'bih' or 'sei' means, and as they might mean something we don't want, we shouldn't just blindly add that.
Rather, you should search for the default static address for image search, without those extra parameters. This can be a bit tricky, and there's not really one way to find it, but for this case, it's 'www.google.com/images?q='
Just as you did before, in the Search engine adding-window add that static part and '%s' on the address, and like magic, you now have a working custom Google Image search! The thing you need to remember that in almost all cases, the parameter 'q=' means the thing you're searching for, so that's what you are looking for.
Instructables' seach bar simply uses Google's custom search, which is a simple code you can add to your website to let people search for stuff specifically on your site, so it's kind of a bad example, but I'm sure you have gotten the idea by now. Basically, it's the same as going to Google and searcing 'something site:www.instructables.com'.
I hope you enjoyed this, as I said earlier I am finnish so my english isn't perfect, and this is my first Instructable.
Please comment if you want me to improve something in this tutorial, or if you find it useful, just say thanks! :)