If you have to pay it now, you have to take out a loan, ask for an advance on your salary or borrow money from friends or family. If it can wait, the amount will accumulate interest until you pay it back. If it's just one month out of twelve, and the other eleven you always have money left over, there's not really anything to worry about. Put the left over money aside to pay this month's expenses. Other than that, there are only two things you can do: spend less, or earn more. Sell your car, move to a smaller place, get a roommate. Cut whatever expenses you need to break even. Try for a second job, a raise, or sell something you make.
If you're running on fumes, financially speaking, but you have some money coming your way soon, consider pawning something of value to borrow fast cash. Of course, to get those items back you'll need to pay back the loan with interest. If you don't pay it back in time, that you'll lose the item. If it's really something that has a lot of intrinsic value to you, don't do it. But if it's something that doesn't, you can certainly consider it depending on your situation.
You could easily do home organizing for people, an industry that has gained a lot of popularity since the debut of Netflix's hit series Tidying Up with Marie Kondo. If you're a tidy and organized person yourself, and you're good at organizing spaces, why not offer your services to people around you? You'd be surprised at how many people, even on your own social media feed, might take you up on doing something like this.
Websites like Care.com connect parents with babysitters. The company does all the background checking and other due diligence to put parents' minds at ease. Of course, you can appeal directly to people in your personal network, but if you're looking to generate recurring revenue sign up with a site marketing to parents looking for child care services.
Use your skills to do day labor. Post an ad online or on a bulletin board offering to do odd jobs or sign up with an employment agency that specializes in temporary work. You can also go where other day laborers meet and wait for employers, such as building contractors, landscapers, homeowners and small business owners. Common odd jobs people need day laborers for include: