how to make dcp

start dcp-o-matic
edit/preferences/default tab
default container=scope
press ok

file/new, enter film name, for example "myfilm-dpc", without quotes
choose folder or create new
press ok.

click on add files button, in the main window
select scale to=scope. Scope is aspect of 2.39:1, which is what people think when they say 2.35:1.
(Apparently 2.35 is not used since the 70's?)

If your input video is 1920x1080,
enter top crop=111,bottom crop=111
This will crop top and bottom a little, and the resulting video won't be as stretched
There is possible a better solution, but this is what I did.

In a destination folder, there will be folder named myilm-dpc_FTR-1-25_S_20_2K_20150830_SMPTE, or similar.
Inside are usually six files, which comprise dcp package.
Resulting video will be about 6x the original size
you can try playing it with ffmplay on pc, but it will be slow.
If the file is small, copy it to cd/dvd, or usb. If it is bigger copy it to external usb disk.
(You may copy original 1920x1080 file to usb, cd/dvd, if you wish, but I believe this is optional, and more of 'just in case' thing.)

If the biggest file is larger than the 4gb, file system on the external disk will have to be ntfs, or ext2/3 (linux format)
WARNING formatting external disk in a new file system tipe, will mean you will lose all your data on that disk.
Make sure you backup important files from external, to internal disk. You can copy them back later, after format.
Be careful not to format your internal disk.

from what I read:
don't use black bars on the side, as the contrast will rarely work, and they will seem gray.

more info
Aspect ratio handling and manipulating: