In the share settings in smb.conf, you'll need to specify the names of users and/or groups that are allowed to write to the share, using a write list = ... line.

Example:

[myshare]
...
write list = my_linux_username

Then you'll need to use the smbpasswd command to set up a password to authenticate my_linux_username for Samba:

sudo smbpasswd -a my_linux_username

This step is necessary because the standard system passwords in /etc/shadow are hashed in algorithms that are incompatible with the password hash algorithms used in the SMB protocol. When a client sends a SMB authentication packet, it includes a hashed password. It can only be compared to another password hash that uses the same algorithm.

(Very, very old instructions from the previous millennium may recommend disabling password encryption in Samba, and using certain registry hacks to allow Windows to emit unencrypted passwords to the network. This advice is obsolete: those registry hacks may no longer work in current versions of Windows, and allow anyone who can monitor your network traffic to trivially capture your password.)

There's one more thing you may have to do client-side. When your Windows client system is joined to an Active Directory domain and you're logged in with an AD account, it automatically prefixes all unqualified usernames with the name of the AD domain of the user, i.e. you will be authenticating as AD_DOMAIN\your_username, not just your_username.

If you are logged in with a local account (or your client system is not joined to an AD domain), Windows may automatically prefix the username with the client hostname unless you specify another domain name.

To successfully log in to a stand-alone Samba server from a stand-alone Windows client, you may have to specify your username as SAMBA_SERVER_HOSTNAME\your_username.

Otherwise Samba will see the username as WINDOWS_CLIENT_HOSTNAME\your_username, conclude that it has no way to verify any users belonging to domain named WINDOWS_CLIENT_HOSTNAME, and will reject the login.

(Newer versions of Samba may have a built-in check for this specific situation, and they might allow you access nevertheless. But this is basically how SMB authentication works "under the hood", and if you need to deal with old versions of Samba, it might be useful still.)

See the Share Definition Access Controls section in the Samba manual. The create mask and related security options should allow you to set the file permissions as you've described.

a good start is to check out the manpage for CIFS:

$ man mount.cifs
[...]
   file_mode=arg
       If the server does not support the CIFS Unix extensions this overrides the default file mode.

   dir_mode=arg
       If the server does not support the CIFS Unix extensions this overrides the default mode for directories.
[...]
   nounix
       Disable the CIFS Unix Extensions for this mount. 
[...]

So since the file_mode (and dir_mode) seem to only work if the server does not support the CIFS Unix extensions, i would start by disabling them (via the nounix option)

If your Windows version allows it, use the standard Windows file permission interface under Properties → Security and grant "Read", "Write" and "Execute" (or simply "Full control") permissions to all items listed. (There should be three; the owner, the group, and "Everyone"). Samba will automatically translate them to Unix permissions or to POSIX ACLs.

A command-line method, one that only works for setting 0777, is cacls myfile /g Everyone:F.

Set the permissions on the directory to be 2777, like this:

chmod 2777 /shared/dir

This causes all files and folders under the '/shared/dir' directory to inherit the permissions of the top directory, in this case 777.

Afterwards, do this to make sure all files have the proper permissions:

chmod -R 777 /shared/dir

Typically one doesn't want to give everyone with network access write access to a directory. There often better solutions to share files among a user group, e. g. giving everybody their own network share with (partial) read access for everybody else. That's where the recommendation to not use world-writable access permissions comes from.

However, in your case that seems to be exactly what you want. So go ahead and use chmod -R a+rwX /path/to/share¹ along with appropriate Samba configuration entries like writable = yes.

¹ a+rwX means that all (owner, group, and others) will receive read, write, and conditional exexute permissions. “conditional” means that execute/traverse permissions will only be added if execute/traverse permission is already granted to at least one access group (owner, group, others). This is useful if you want to extend execute/traverse permissions of those files and directories that are already executable/traversable by the owner.

It's not clear to me if you want a share accessible to everyone or only a group of client users so this is a template for the latter.

Note: This will only work as described if the default umask of your system is 0002 so it will not work for Ubuntu Desktop 17/18 but it will work for Xubuntu Desktop and Ubuntu Server.

In this template it is required that all users you want to have access become members of the "users" group. It will assign the setgid bit on the shared directory and any future subdirectories which forces anything new added to them to inherit the group of it's parent folder.

sudo mkdir /path
sudo chown root:users /path
sudo chmod 2775 /path

The share would look like this:

[UsersShare]
path = /path
valid users = @users
force group = users
read only = no
create mask = 0664
force directory mode = 2775

When bob - who was made a member of the "users" group - logs in with his samba username/password and adds a file to the [UsersShare] share it will have owner = bob, group = users, mode = 664 files / 2775 folders.

All other client users who are members of the users group who sign in will have full access to whatever bob did.

Any local users - those on the server itself - who add or modify files who are also members of the users group will all have the same ability and newly created files/folders will have the same 664/2775 mode and all will have as group: "users". The one exception to all this is root - as in when you use sudo - since it's default umask is 022 unlike a regular user his files will inherit the "users" group but will have a mode of 644.

If you truly want a pure public share where everyone has access replace "valid users = @users" with "guest ok = yes". For this to work locally on the server you would still need to add those users to the "users" group.