The linked article you showed in your question gave good and pretty specific instructions: flatten transparencies completely, convert all spot colours to CMYK.
I'm not going to go through colourspace discussions, but I'll quickly (heh!) take a screen-targeted illustration I did to answer someone's question about blends and softening effects and flatten it, knowing as I choose this one it'll be a real beast for Illustrator to flatten, 'cos it has multiple complex transparency effects, glow and feather effects and all in multiple stacked layers so it should represent a pretty darn serious challenge for Illustrator to process through all that jive.
- Unlike what I am doing (on purpose) here, I would typically flatten sections as a time of your design to avoid Illustrator lagging so darned hard.
- Flattening Transparency by its nature is a destructive process: make a copy of your final working file and flatten and convert spots to CMYK in this Print Delivery Only file, leaving your final design file undamaged.
First select the items to flatten, then choose Object>Flatten Transparency:
This will bring up the Transparency Flattening dialogue box, where you can set specific parameters for how you want to flatten your image.
Once you commit, be aware this will take a long, hoary time to complete:
And I do mean a long hoary time:
When I said a long time - we're now at almost ten minutes:
After Flattening completed (you can see blends exploded and all transparency effects converted:
Image with no elements selected - the flattener did a great job maintaining appearance; this file would now be printable with minimal issues, whereas the design-final unflattened file wouldn't have printed at all.
After this, you just save your now enormous file as either a straight
.ai or a press-ready
.pdf depending upon your printer's published or stated preferences, and you should be good-to-go - and BTW - when I say now enormous, in this file's case we went from 4.49 Mb to 391 Mb in just flattening - so you'll want an FTP or other file transfer solution already dialed in!
Hope this helps!