I think it is dangerous to look at it from a 'top 5 options to gain steam' point of view. Those are tactics that can help, but it is extremely important that you first define a strategy behind it.

To significantly increase your chances of success (and avoid extreme overwork trying 5 tactics at once, which often causes burnout), I highly suggest you do two things :
1 - select a specific industry
2 - select a specific geographic area

The best parallel I can do is with Starbucks. It looks today as if they are everywhere. But in reality, they started small, tested and tuned their 'system' locally, and gradually expanded geographically.

Facebook did the same (started by only focusing on the academic market, on one specific campus, with a clear focus on the 'dating' aspect, and then expanded).

A clear message is very important in terms of customer acquisition because a high level of specificity creates an 'this is for me' reaction, which instantaneously creates trust in your solution (one size fits all is dead).

I see many dangers in your sentence "Help businesses post their services to reach customers".
To significantly increase your chances of success (and avoid extreme overwork which often causes burnout), I highly suggest you do two things :
1 - select a specific industry
2 - select a specific geographic area
The best parallel I can do is with Starbucks. It looks today as if they are everywhere. But in reality, they started small, tested and tuned their 'system' locally, and gradually expanded geographically.
Facebook did the same (started by only focusing on the academic market, with a clear focus on the 'dating' aspect, and then expanded).
In your case for example, your statement could be "Help accountants in Boston post their services for free to reach a larger consumer base".
Notice how you get instant focus with this simple change, you now know who to reach on both ends.
You can get even more focus by using a specific business size and smaller geographic area :
"Help accountants doing over 5M$/year in Cambridge post their services for free to reach a larger consumer base".
I clear message like this is very important in terms of customer acquisition because this level of specificity creates an 'this is for me' reaction, which instantaneously creates trust in your solution (one size fits all is dead).

First, congratulations for the new product!
Can't tell you about the 'where', but as per the 'how' I would suggest you adopt the 'feedback loop' mindset where instead of pushing to sell your product, you do all you can to involve your potential customers in the product development process.
The key psychological element here is that by getting their feedback and actually implementing them, you create a sense of 'belonging' in the marketplace toward your upcoming product, which will ultimately be of massive help during your launch.
In other words, don't approach the market as if you had a finished product, approach it as if you had a rough first draft you want them to improve (note that this is very hard to do, you have to put your ego on the shelf ; ).

Absolutely, now more than never!

If you take aside the psychological factors (motivation, strong desire, etc.), business success in general is dependent on 2 key variables : money and time (*not* experience).

You cannot start a business with no money and no time.
But if you have either of the two, you can make it happen.

As per how, with all the dogmatism of brevity I would say:

The easiest is to create a service that you can learn and deliver yourself (zero cost, only requires time to learn the required skill - could be writing content, teaching a language, coding apps, walking the dog, etc). Check out craigslist for about a thousand more ideas.

Even for a product, you can ask your first customers to pay for it in advance (to cover development costs) so you get the funds to pay tech peoples to create it.

Either way, there are 3 skills you must master if you want to have success :
- get good at asking questions to find where people's problem are (the seed of any business is in resolving a problem, so you got to stop talking and listen!)
- get good at asking them what their ideal solution would look like (you would think that people already looked for a solution if they already know what it looks like, but this would be over-estimating human behavior - you will find many were not even *aware* they had a problem before you started asking)
- get good at hustling to put this solution together for them and then sell it!

One word of caution : the real challenge in starting a business is in finding a profitable niche that can sustain you over the long term doing work that you love.

To achieve this, you must approach it the same way a child learns how to walk. By falling down, and trying back again.

If you want a fantastic example of what this kind of hustle can create, look no further than Clarity's founder Dan Martell (check out the 'About Me' page on his website for a great example of the "entrepreneurs' journey")

To get the proper mindset, I also suggest a total immersion in the entrepreneurial world : listen to podcasts (free), read blogs (also free), and invite entrepreneurs for lunch to pick their brains (ok, that one is 20$).

The previous answers are really excellent.

Let me resume them quickly and add a fourth point which is critically important to help you putting all this together :

1 - keep in mind the psychological aspect to make sure you reach your goals : a book has a much lower value in people's mind than a training program, and for a pdf it's even worst - people are used to get pdf for free!

To break this psychological link (pdf = low cost = you will never make much money out of it), consider creating a multimedia training program including both videos and the pdfs.

Higher up in the 'value scale' is offering group coaching, and even better one-on-one consulting.

2- Be aware that the key thing your customers will be looking for is PROOFS. With so much hype all around, this is THE thing that will "make or brake" your goal achievement.

So if you already implemented this strategic planning methodology yourself with great success, that's enough proof to get started.
Otherwise, consider offering your methodology for free to a customer in exchange for a testimonial (proof).

3- Either way you go with this, offering part of your methodology for free is an excellent proof element and should be used as part of your marketing to generate leads.
You can do that by sharing a pdf (or by doing a webinar).

4- Note that the real question here is : what business do you want to create for yourself?

If you create your own digital product (pdf or others) and service line, you will have to manage the web 'infrastructure' and marketing but you will then have direct access to the customers (which is the real asset in your business, NOT your product).

If you create partnerships for others to sell your product, you may make some money, but you will never be in contact with the end customer - hence you won't have any real asset to grow a potential business on.