Why 说的 and not only 说 in 你要按照我说的做并且要做好?
In Chinese, noun phrases in the form:

nominal modifier + 的 + noun

may drop the noun when:

it's clear enough from the context
it's a term very generic in meaning

Clear from the context
This is the case where a noun is dropped to avoid unnecessary repetition.

这本书很有意思，谁写的（书）
This book is very interesting, who wrote it? (Who wrote the book)

Very generic in meaning

你要按照我说的 (话）...

As in your case, the generic nature of the noun may be given by two factors:

it's actually a generic term, as 事情 (matter)，东西 (thing), sometimes 人 (person, guy). It can be translated in English as a relative pronoun, e.g. &quot;what&quot;:


他建议的（事情 / 东西 / 意见），我不太了解
I don't really understand what he's suggesting


昨天我遇到的（人）就是陈杰最好的朋友
The person / Whom I met yesterday is actually Chen Jie's best friend


it's the object of a separable verb (because it has a generic unspecified meaning)


我每天吃的（饭）是我娘做的（东西）
The food / What I eat every day is what my mum prepares