[Choose Your Flavour] In the dialog-box, start by clicking on the 'General' tab, and selecting PDF/X-3 from the drop-down menu for 'Compatibility.' It's a good idea to Downsample images to 300 dpi, at the bottom left here.
[Color Management] In the 'Fonts' tab, choose to embed all fonts and 100% of all characters. Then click on the 'Color' tab. Note you can use a source ICC profile for the file, choose its rendering intent, and also choose whether to use the source profile of the Images, or to discard it and choose your own. If you don't understand this, don't worry. It is quite simple.
Head out to littlecms.com and read the documentation available there. Also read the documents on the Scribus website, and check out the links to the excellent reference material on Scribus' page.
[Press Inks] Finally, click on the 'PDF/X-3' tab. You have to choose the output profile, which is basically selecting the kind of ink and paper you are printing your magazine on, and giving the job a mandatory 'Info String'. Give the name of your file here. You can also handle some amount of trimming, which is changing an existing paper- size in a document to a slightly smaller size to accommodate a print-job.
Before sending your job to the press, please go to Extras> Print Preview. You can see a soft-preview of your file in the inks of Cyan, Magenta, Yellow, and Black, and even simulate a progressive proof on-screen.
[Native Scribus File To Press] If you are blessed to have a Scribus-savvy printing plant, and wish to deliver a native Scribus file, go to File>Collect For Output. Select a folder and Scribus will copy your file, as well as put copies of all graphics and images, and even font files, in this directory. Transfer this folder to your press. May I recommend you throw in a copy of Scribus in this folder too. A clueless person can discover the joys of Scribus. Go ahead, this is perfectly legal, thanks to the power of GPL.