This depends on the type of offering a company is making. If it is a CF Offering, which is most common, you will need to register with the website the company has chosen. Most common of those are Republic, WeFunder, and MicroVentures. After registering, you need only follow the directions and fill out the information to invest.
For other offerings, a 506(c), for example, the company will create their own investing web portal and will have all the relevant investment information on that page for you. We will direct you to that page in such cases.