Install Directory Server from the package center, this will provide the LDAP server on Synology devices. In Settings, enable it as the "Provider Server" and input a fully qualified domain name (in our case the local IP address) and a password. This window provide you with the authentication information (base and bind domain names) that will be required for client-side configuration.
There is a default admin account and several default groups, so you just need to create LDAP users as members of the users group. Only user name and password are mandatory, but you can input additional information as well. In the Advanced tab you have also the option to enforce some password rules (recommended, without excessive strictness).
These LDAP users will not appear as local users on your Synology device and do not yet have any allocated disk space. For them to be recognized, we first need to enable the LDAP client in Control Panel, Domain/LDAP (note that this is incompatible with joining a Windows domain). The server is on the loopback address 127.0.0.1, encryption is SSL/TLS and the base DN is the one obtained in a previous step.
In the LDAP Users tab, select User Home and enable home services for LDAP users. This will create home folders of the form "/var/services/homes/@LH-domain/num/user-uid" that will be mounted by the LDAP/NFS clients.
One last thing to be done: indicate through LDAP the mount point that will be used on the clients. This is not possible through the standard LDAP configuration interface on Synology, for this you must download phpLDAPadmin and install it in your web folder (assuming you already have Web Station and PHP running - do not forget to activate the LDAP extension for PHP). Open it in your web browser and, for each user, modify the homeDirectory setting to reflect the proper mount point (in our case /mnt/nfs/home/username). Note that phpLDAPadmin gives you a fine-grained control over other LDAP parameters if needed.