First off, you need to figure out the path of your flash drive:
fdisk -l
Then… edit /etc/fstab as follows:
/dev/sda1 /mnt/flash vfat rw,user,umask=0002,uid=1001,gid=121 0 0
Note that umask is… the “inverted” regular file mask. This represents 665 (because we’re evil). As per uid + gid, you can figure it out by means of this command:
id username
Update:
The best filesystem to use, if you need to maintain compatibility between the Flash Drive you’ll use with your Raspberry, and macOS, is probably ExFat. Now, problem is: Linux doesn’t really support ExFat by default.
So, we’ll need to install it, as follows:
sudo apt-get install exfat-fuse
In such case, your /etc/fstab file should look like this:
/dev/sda1 /mnt/flash exfat. rw,user,umask=0002,uid=1001,gid=121 0 0