I'd have written it as:
When I worked as a technical writer I mostly wrote Mil-Spec manuals for the US Military. I had an excellent Style Guide which I seem to have misplaced. I'll find it eventually. It was the Style Manual (Guide?) for the US Government Printing Office.
Sorry, but Ernest's edits to remove two commas are absolutely correct, at least for a writer of fiction.
You could have commas in those two places, but they would be really stressing the following clause is very important.
It took me a lot of effort to find that many of the commas I was using early on where incorrect.
Firstly, my ideas were being corrupted by the only references I could find (those damn Style Guides), which insist commas are needed in various positions that really only apply to technical writers. In fiction they interrupt the readers natural flow from one idea to the next. When revising sentences to eliminate excess words I often conclude a sentence with one extra word, but one extra comma in the alternative, is better left alone.
Secondly, for some time I had the mistaken impression that the purpose of punctuation was to guide the reader where to make minor pauses while reading. That is not so. They do that for themselves from the context.
Ernest's explanation did not use correct terminology, but he was spot on with his concept of learn how to recognise what constitutes a "sentence fragment", and then learn the valid ways of joining fragments.
Commas can become complicated because they have several types of uses. They are used to separate both elements of lists and fragments of sentences. They are also used to separate a series of adjectives. Elements of lists may be anything from simple words to complex clauses. The thing (or one thing?) that makes them a list is that all have equal status in the way they relate the the introduction of the list. The important thing here is to decide whether you prefer to use serial (Oxford) commas or not, and then be consistent in using your choice.
What constitutes a sentence fragments can be a bit tricky. There are a few that must ALWAYS be validly separated from the main body of a sentence.
1. Dialogue and all attached attributions. For example: "Stop that!" she yelled furiously and then hit him.
The dialog fragment there ends with "furiously", because the attribution includes the subject and "speaking" verb and any adverbs or adjectives modifying them.
2. Interjections always constitute a separate fragment. That is any extra words just thrown in there with no connection to the main sentence, i.e. the rest of the sentence is unchanged if you remove them completely.
3. All kinds of asides, comments and explanations. If your sentence makes sense if you enclose it in parentheses, then you may use something else (often commas) to separate from the main body of the sentence – but you must use something.
4. There are also independent clauses. That is anything that makes sense if you place a full stop after it.
These are the types of fragments that must be separated, and the first rule is then the need or comma (or something more) unless they have a coordinating conjunction. The mnemonic to remember the most commonly used coordinating conjunctions is BOYFANS - and plus but, or, yet, for, as, nor, so - but anything else that means the same as well. Note there are eight common coordinating conjunctions and they all have only two or three letters.
I will not go on to describe introductory clauses and non-restrictive clauses which also require commas in technical writing. In fiction that is not mandatory. I rarely use them for introductory clauses, but usually try to for non-restrictive clauses when I can spot them.
Also, I cannot guarantee everything I've described here is totally accurate – but it is definitely quite close.
I apolgise for directing this lengthy lecture at you, much of which I am sure you do already know.
My MAIN REASON for this post is to describe the tool which helped me (finally) understand most of these concepts and get them right most of the time.
I downloaded and installed the free software from grammarly.com. It really took no more than clicking on the obvious button half a dozen times. Without doing any more it began checking the grammar of my emails as I was typing. [I think there is an extension for Word, etc., but I have not bothered with that].
In only a short time I can see the clarity and correctness of my writing has improved substantially.
I VERY HIGHLY RECOMMEND THE FREE PRODUCT TO OTHERS
There are two cautions I suggest to authors of fiction:
1. I almost routinely dismiss its flags for 'comma required after an introductory phrase'. Provided the idea in that clause flows naturally into the main body of the sentence, I let my readers read without insisting they make a pause.
2. I usually, not not always, accept its flags for 'comma unnecessary in a complex sentence', but sometimes leave them so that readers will pause because the next idea is important.