I am not an expert in Black Speech, so I'm going to follow the information from the blackspeech.ru website.
The basic word order of Black Speech is SVO (subject-verb-object), as in English, so the subject is Sauron, the verb is watch, and there is no object. Adverbs (always) are to be placed after the words they modify, so we end up with
Sauron watches/is watching always
as the basic starting point. Sauron is straight forward,as there are no cases. With watch I would actually choose the translation tud-, as it has the sense of guard, which seems more appropriate here; in the third person singular this would be tudat. The adverb suffix is -arz, which gives us ûkilarz. So our sentence would be
Shakhbûrz tudat ûkilarz
With the Tengwar transcriber on the same site it would be rendered as
UPDATE: In case you want to say "Eye of Sauron", that would be the possessive ("eye Sauron-of"):
Hont Shakhbûrz-ob tudat ûkilarz
There are no articles, so it's either The Eye of Sauron or Eye of Sauron — there is no distinction in Black Speech.