The first thing I did was install DosBox on my desktop. That way, I familiarised myself with how a DOS emulator works. You can follow the tutorial at the DosBox wiki to get you started. There are plenty more tutorials and videos available online to help.
rpix86 is the faster emulator as well as being the default emulator in ES so you may want to try this first on your pi. You need to place your unzipped game folders in the ~/RetroPie/roms/pc folder and they should appear in ES. Then check out the tutorial for using rpix86.
FastDosBox is an optimised version of DosBox for the pi so it should run in the same way. If you want to try it, then you will need to change the shell command (<command> section) for PC(x86) in es_systems.cfg to
<command>/opt/retropie/emulators/fastdosbox-1.5src/dosbox</command>
Bear in mind, I have not tested the DOS emulators in Retropie. I am still playing around with them in Rasbpian so I can get to grips with how they work before I’ll be happy to introduce them to my Retropie build.