Change Color Mode to CMYK - flatten image?

[HelloFromCat]

Hi. I'm new to PS. For an assignment I need to deep etch an object, with the color mode converted to CMYK. I'm on a PC, lecturer on a Mac. When I go to Image>Mode to change the color, I get a pop up asking if I want to Flatten the Image. Will this affect the deep etch? - do I do the deep etch first and then convert, or convert and flatten before doing the deep etch?

I've googled to try and find the answer but am coming up short. Thank you. 

[D Fosse]

You have to flatten. Layers can't carry over from one color mode to another - they will either mean different things, or not be applicable at all.

A simple transparency will carry over though. I suppose that's what you need.

But I must say this is a bad assignment for beginners. CMYK is for commercial offset print only, and it is definitely not for beginners. You really need to know what you're doing.

To begin with, there simply is no such thing as just "CMYK". CMYK is always tied to a very specific offset print process, it's never generic. You always need to know which CMYK profile to use. If you use Image > Mode, Photoshop's rather arbitrary default CMYK profile will be used, and it will almost certainly be the wrong one.

[HelloFromCat]

Thank you so much - I hadn't even yet comprehended what Flatten meant. I'm not sure why we have to convert it to CMYK as we're not printing anything, but the assignment is to create a magazine layout, so I guess it's mimicking what the actual process would be. Everything I've read though, says you need to check with your printer to see what their preferred CMYK is, rather than just going with the default like I'm having to do here. Thanks again.