Hi, for this open /etc/samba/smb.conf and add + adjust an additional block to the end of the file, that looks like this:
[myshare]
comment = my share
path = /mnt/myHDD
browseable = yes
create mask = 0664
directory mask = 0775
valid users = dietpi
writeable = yes
max connections = 1
The name in the square brackets is what needs to be used when adding the share to a remove Samba client, the comment is more like a visual name. Depending on your requirements, authentication user and permission modes can be adjusted of course, the above example matches the default dietpi share accessible by the dietpi user and files+dirs are created a way that all members of the dietpi group have write access as well, e.g. download and media software.