Capital is created by those who borrow when they issue IOUs, as those liabilities (from the point of view of the firm) are by definition the assets backing the company (from the point of view of those who have purchased those liabilities from the market and hold them as assets). E.g. the stock of a company is the liability of the company but an asset for the shareholders that are funding the company.
But not just any IOU can become an asset. Only "marketable" IOUs have this property as the IOUs should be priced as they circulate in a market, at least conceptually. It is the ability to circulate that gives the IOUs market value, and this market value can be exchanged for the necessary inputs to fund the venture. People accept these IOUs as a form of payment because they can in turn sell them to someone else for whatever they themselves need, again because the IOUs are marketable.
Thus there are two critical pieces: the issuing of the IOU by an entrepreneur and then the pricing of the same in a capital market.
Supposedly you are asking what market institutions exist to price newly issued claims, which is a question of having accountancy, venture capital, and other skills that would allow one to help estimate cash flows in a credible manner. Creating such institutions is not simple, but once they are created they are invaluable as they allow for the creation of unlimited capital[1] to fund entrepreneurs, provided that the credibility of the market instruments remains intact.
[1] Of course while the ability to issue promises of future profits is unlimited, the actual availability of profits is certainly limited by things like GDP, the rate of innovation, the availability of inputs, etc. Thus only some promises of future profits are actually realized in a market, and it is the job of those who price claims to, on average, apply appropriate discounts to these promises in order to, on average, accurately price the claims.
