That sort of depends on what software you are using to do the job. I have created several bootable CDs using ECDC 5. Whatever you use, open the User Guide and look in the index for "Bootable CD - making, etc." This is the gist:
1. Make sure your BIOS supports booting to CD ROM.
2. Insert a blank CD into your burner.
3. In File Menu, select New CD Project, then Bootable CD. A "Choose Type" of bootable CD dialog box appears.
4. Select type of floppy disk emulation - normally 1.44 MB, or HDD emulation if you want many programs, etc. on the CDR.
5. After selecting the type, then follow the instructions as they appear. You will normally need a bootable floppy from which to obtain the boot files. These will be placed in BIN files on the CD.
Other software will vary, but the principle is the same.