The sentence does not have an introductory clause. Rather, it consists of two independent clauses:
- I could do this myself
- I suggest you follow the manual
Note how each one could be a complete sentence on its own. Joining them requires both the comma and the conjunction ("but").
An introductory clause, in contrast, is usually a dependent clause that begins with an adverb (see the Purdue Online Writing Lab for more examples). Rewriting the sentence to use an introductory clause might yield:
Although I could do this myself, I suggest you follow the manual.
Note that the first part of the sentence can no longer stand on its own, and there is no conjunction, but the comma is still required.