Here's the output from apt-cache search ltsp on raspbian (Debian Wheezy compiled for the armv6 raspberry pi):
fts-ltsp-ldap - LDAP LTSP module for the TFTP/Fuse supplicant
fts-opsi - LDAP LTSP module for the TFTP/Fuse supplicant
ldm - LTSP display manager
ldm-server - server components for LTSP display manager
ldm-themes - Collection of themes for the LTSP login manager
ltsp-client - complete LTSP client environment
ltsp-client-core - basic LTSP client environment
ltsp-controlaula - classroom management tool - LTSP client version
ltsp-docs - LTSP Documentation
ltsp-server - basic LTSP server environment
ltsp-server-standalone - complete LTSP server environment
ltspfs - Fuse based remote filesystem for LTSP thin clients
ltspfsd - Fuse based remote filesystem hooks for LTSP thin clients
ltspfsd-core - Fuse based remote filesystem daemon for LTSP thin clients
The ltsp-server-standalone package is mentioned here "if you want a complete LTSP server with all the bells and whistles". So the answer to your question, "How to use ARM as LTSP server?" is: Exactly the same way you do it anywhere else. They even have a wiki.
That said, if you are looking to use a raspberry pi, or a banana pi, or any other low power ARM system for this purpose, you are almost certainly going to be disappointed. These machines can support a desktop for themselves, but they simply do not have the power or the RAM to do it for multiple clients. I could be wrong of course -- good luck!