@David_Cooper -- That, indeed, is the right question. AAAaaaannnnd ... the answer is simple, but you won't like it.

You start by getting a job at an ad agency and learning how the business works from the inside. That might take some time, but understand that is how most agencies came into being. Some employee or officer of an agency thought, "By damn ... they have very few good ideas - most are so bad that all they do is make people turn the sound off until the programme comes back on. I should know, because I've practically worn out the mute button on the remote and have to press it with half a ton of force to silence the damned adverts." He or she then trundles off to start an ad agency that he or she is convinced will put all the others to shame when his or her agency alerts the public to the fact that the others have so few good ideas.

On a more serious note, the advertising business is complex and it takes a while to learn what is necessary to know. The process is further complicated by the need to establish contacts in all the right places with all the right qualifications. The starting point, if you don't want to work in the field and self educate is a bachelor's degree in advertising, communications, marketing or a related field that can provide a foundation in the creative, management, interpersonal and analytical skills required to work in the field. Still interested? Have at it.

@David_Cooper -- Your decision. All I can say is that I've been in and around this business since 1956 and I've seen a lot in that time. If you have the time, energy, drive, commitment, and funds to do it, give it a go.

Did that for years, doesn't work, they just claim simultaneous creation.
Self publishing first does work, they won't go up against it, which is why most places won't accept scripts already self published, those who have no intention of stealing from you will.

@LenHazell53 -- That is not why publishers don't generally accept published works. There are some that will publish reprints and there are a few that will accept work published in obscure places where there has been minimal exposure, but they are rare. The real reason deals with exclusivity/first rights in the case of material that is not bought outright. For example, there are many short article/story/essay publishers where the rights for a work become the property of the publisher and the author can never sell the work again. The majority of anthologies are handled in this way.

This exclusivity idea goes back over a century and is not some conspiracy designed to rob writers of their work.

@evidentialist I never said it was a conspiracy, bit everything you have said does not apply to self published works, but is applied anyway simply because it is proof of ownership of the intellectual rights.
It is also however a way for publishers to see if works they would never have published will sell, and so can be pounced on E.L. James is proof enough of that.

@LenHazell53 -- Oh my.

I resent that, Len. On the other hand, his request has nothing at all to do with editors, or sub editors. It does have a lot to do with ad execs.

@evidentialist Resent it all you like, it happens A LOT, people hired to come up with new ideas can't so steal speculative scripts and idea sent to to keep up their reputation as miracle workers.
I have a rejection letter for a script I wrote in 1988, signed by a then junior script editor, thanking me for submitting to Granada Television but informing me regretfully it would be too expensive to produce.
That Editor is now a highly powerful writer and producer at the BBC and his break through series was my script, with another title, even some of the characters have the same name.
I took legal advice and was told it is pointless going up against the BBC they have a whole legal department for dealing with such claims, IT HAPPENS SO OFTEN they hire people to deal with it specifically, and yet the BBC and most other UK TV stations still operate an open door policy for writers to submit work to them unsolicited.

@LenHazell53 -- I'm responding to the use of 'all' in your comment. We are not 'all' similes for thieving scumbags. If that were true, I would be considerably more prolific than I am. Of all the work submitted to me, only a bit over 10% is accepted. That acceptance rate is common throughout the industry and I daresay with the amount of material I have submitted (substantial) I would have experienced such theft at least a few times by now. I've never had that problem. There is a problem with pirate publishers selling my work without paying me royalties, but that has been with us forever.

What ms was stolen from you by the BBC?