How to make a DOS bootable flash drive

Ok I got it working with an old IBM Thinkpad. Here are the steps I took:

Get DOS Image File from Here:  WinWorld: MS-DOS 6.22 (winworldpc.com)

Get RMPrepUSB Here:  RMPrepUSB download latest version (fosshub.com)

Get the Dos6.22 IMG file from here as you already did.
With RMPrepUSB (I'm using v.2.1.739) select File->Drive.
Go through the prompts and let it put all the files from the ISO onto the USB stick.
Copy ALL the files on the USB stick to somewhere on your hard drive (these should be the only files in the folder).
"In the Copy OS files from here" section, put the folder that you just copied all the files from the USB stick to.
In Sec.3 select MS-DOS bootable.
In Sec.4 select FAT16 on the left, and "Boot as HDD" and "Use 64hd"**
(Make sure the checkbox next to "5 Copy OS files" is checked)
Select 6 Prepare Drive and follow the prompts. It will partition, format, MBR, bootsector and copy the files to the USB stick.
When 9. is complete, hit Eject Drive and remove USB stick.
That's it! Let me know if it works for you.

Note: I think the problem with just doing the File->Drive thing is that it doesn't put the files in the right place on the USB stick. When you let it prep and copy the files itself, it puts everything in the proper place and order.

** You may have to monkey with the checkboxes in this section. My laptop places the USB Boot under its "Hard Drive" section when it lists it in its BIOS. Your computer may consider it removable storage, or a FDD.