Writing a good Khmer poem is difficult because the poet must be proficient in the Khmer language and the strict rules he needs to follow. These two prerequisites must be met if you are serious about writing Khmer poems. It would be easy to learn the rules if you are already proficient in Khmer, and writing would come naturally. However, if you just learn the rules and you lack the command of the language -- it'll be harder.
This web site attempts to teach you the rules and nothing more. If you want to improve your poetry writing skills, you have to read the online examples elsewhere. Better yet, read the poems by famous Khmer authors such as, Keng Vansak (Kuk Karm Kiless), Oknha Sottan Preyjea In (Nireas Norkor Wat), Koy Sarun (Bopha Kmao, Kert Jea Monus Niss), Somdech Chuon Nath, Phikhok Saom (Tum Teav),etc. This web site provides you some examples but it's not enough. And I don't claim my poems to be good samples because I myself am still learning.
If you just want to write for fun and less concerned about whether your poems are any good or not because you want to express yourself in Khmer by posting on poem forums at khmer.cc, khmerfuture.com or camweb.org, learning the rules will equip you to do just that. There's no such thing as a free form in Khmer poetry. If you like to do this kind of thing, in my view, you should stick to writing in English.