Install FreeBSD usb drive

I had FreeBSD on an old computer but the laptop fried. I want to install it to a usb drive and boot into it. Almost everything I've found is about using it for installation on the computer and that's not what I want. I downloaded the memstick.img but I'm not sure what flle system to use on the usb stick. They come with fat32 from what I remember. I tried one of the dd commands that was supposed to work but it wouldn't boot. I don't want to install it on my new computer because I think my hardware would be too new. Plus it should be easier.

I have seen this problem too, when creating a USB based installation. Installing from an USB memstick to a USB device.

The CD/DVD installer will work with no issues.
Use labels, then it won't matter in what order or how many USB sticks you have. Installing to USB works in exactly the same way as installing to an HD. Both are represented as disks and are therefor functionally the same. You only need to take care you're installing to the correct USB stick and don't inadvertently overwrite the installer stick.
The NanoBSD reads like it must be done from within FreeBSD. I checked out the labels but I'm confused. I read that the dd command could be used and also that there was an option to install to usb during an installation. II didn't see that so I quit the installation. I've installed from a usb stick but never installed onto a usb stick. Sorry for being dense.
Lets toss out the whole nanobsd suggestion.
Like you said with a FreeBSD memstick installer you can pick any drive you have attached. So lets say your FreeBSD installer resides on one USB stick, The system will call this device da0. Your second USB target device device would then be called da1. FreeBSD can freely install to da1 and it works. The problem is at the end of installation you reboot and remove installer disk. Now all of a sudden what was once da1 is now trying to mount at da0. Failure. This is by design.
So the suggestions here are showing you how to deal with the devices 'label' to successfully boot.

Another way would be to edit /etc/fstab at the post install shell.