SSD's directly take the place of the HDD's, no problem. Just unplug and plug the same SATA data cable from the HDD to the SSD (any power cable can be used then). It will use the same drive letter, unless the HDD is partitioned, the extra partitions will disappear.
That's the rule of the thumb. However, if it is a system drive, the system must be cloned to the SSD first (could be done even thu any external case that fits and sometimes it's provided with good brands) and the it can replace the system HDD as above. Sometimes this works out of the box but some other times bios changes must take place to avoid blue screens and such, and some other less frequent times the OS needs to be reinstalled to accommodate the SSD difference. I did that once and I needed only to change some bios settings. The OS could ask for a repair disk so keep that at hand.
Boot order should be controlled from the bios, but some inferior designs don't have it and cables have to be tampered with. But recognition should still be the same regardless.
Having the optical disk drive (DVD, CD, etc.) as the first boot drive is best and probably required if booting from them is something you do, so having them 1st is the way to go. In some cases it slows the boot but that's probably in the old days and not anymore. If they are not, then the system will check prior drives for boot files and only go to the optical drive orderly if it does not find any.