If it's plain OpenLDAP
The libpam-ldapd route is the least painful because the installer prompts you interactively and auto-configures nsswitch.conf + PAM:

sudo apt install libnss-ldapd libpam-ldapd

When it prompts, just fill in:

LDAP server URI: ldap://your-server:389
Search base: dc=example,dc=com
Check passwd, group, shadow 
Then set the bind DN/password:

sudo nano /etc/nslcd.conf

binddn "cn=readonly,dc=example,dc=com"
bindpw your-password

sudo systemctl restart nslcd
sudo reboot