https://www.youtube.com/watch?v=ZNaT03-xamE

lsblk

to view media. Let's say your flashdrive is in 'sda'

fdisk /dev/sda

to wipe it. Enter d for delete, then n for new partition, then p for primary, and hit enter to accept the defaults and type YES

lsblk 

to look again. Now you have a sda1

cryptsetup luksFormat /dev/sda1

to encrypt it. Select a password for it

cryptsetup open /dev/sda1 drive

to open it and name it, for the time being, 'drive'. Enter the same password you gave it

lsblk

and you should see 'drive' there

mkfs.ext4 /dev/mapper/drive

to make a filesystem on it (because it doesn't have one yet). it'll auto be on mapper. It'll take some seconds or a minute

mount /dev/mapper/drive /mnt/

to mount it (to mnt for the time being)