It's quite simple to create opensearch descriptions for any existing Google CSE. Here's an example using the mozillaZine KB CSE:
This CSE is identified by cx=003258325049489668794:ru2dpahviq8. The &cx=-parameter is used in 
links and anything else related to this CSE. The left hand side 003258325049489668794 is related to the Google account and the up to 5000 annotations (e.g. sites and URL patterns) associated with this account. The right hand side ru2dpahviq8 is related to the actual CSE context including details of its layout, references to the associated annotations also known as background labels, etc.
It's not my CSE, I can ignore most technical details only relevant for the CSE creator. One detail is probably important, this CSE uses FORID:1 unlike my own CSEs with FORID:0. The value is visible in the monstrous URL of search results, it's a part of the &cof= parameter.
Most other layout details noted in &cof= are set by Google on the fly based on the CSE definition a.k.a. context. For my own CSEs I force LP:0 and AH:center with &cof=FORID%3A0%3BLP%3A0%3BAH%3Acenter, but that's arguably pointless, opensearch only works with Firefox 2, IE7, or better, and these browsers have no issues with the default LP:1 logo position and AH:left aligned header on result pages.
CSEs refuse to return &output=xml or &output=xml-no-dtd results, therefore the opensearch description needs only one type="text/html" template. Just in case I added...
<Attribution> Google CSE by Jason Kersey </Attribution>
...anyway, after all the search results are Google results. In this case results filtered and rearranged as defined by the CSE creator Jason Kersey. With up to three searched sites in a CSE Google allegedly also shows its supplemental results.
Putting it all together I arrived at this mozillazine opensearch description. I've no clue how and where Firefox or IE7 might use the Tags or Description, most likely these details are irrelevant for search results on ordinary type="text/html" pages. The validator wants a Query example as specified by opensearch.org, just for fun I picked about%3Aconfig.
One last detail, the icon, fortunately kb.mozillazine.org has a type="image/vnd.microsoft.icon" 16×16 favicon needing less than 10 KB, so this should work as is (http:-URL instead of data:-URL) for Firefox. It's tricky to get the icon right with *.googlepages.com, the Google Page Creator won't let you have your own favicon.ico. Just use another name.
One way to use opensearch descriptions is to add a link in the header of (X)HTML pages. The title in the link should match the ShortName in the description, otherwise browsers won't know if the corresponding search is already installed. I've done that here in my blogger-template:
Another way is the window.external.AddSearchProvider function, Firefox 2 users can then simply click on the link to copy an opensearch description to their browser. I haven't tested IE7, maybe it uses the same method, i.e. "copy description". Last step, test this OpenSearch description with Firefox 2 or better.
For another example see my googlets page.
