I'm a Mujeer. This is not the quick answer, nor the long answer (I've written a blog post How to Become a Forbes Writer with that). But in a nutshell, here's my recommended method, assuming you don't have a personal connection at Forbes:
Create a body of great writing that could easily qualify as Forbes quality.
Reach out to contributors at Forbes and offer help in the form of quotes, stats, etc. based on your expertise.
Once you've been quoted in Forbes a few times, then contact Forbes directly and give them your pitch from the stance of "I've been quoted in Forbes already, and here are some samples of my writing."
Love writing. If you see becoming a Forbes contributor as $$$ or fame rather than something that will fulfill your passion for writing, don’t bother. It will become a burden rather than a blessing. The majority of those who thrive as contributors genuinely like to write.
Write a lot. Showing that you’ve written 3-4 blog posts isn’t enough. Showing that you’ve written 100 blog posts, and several pieces that have been published elsewhere, is much better. One of the things Forbes wants to know is whether you will produce a piece every week, on average. That’s 52 articles per year. It’s a lot of work. What evidence can you show to prove you’re up to the task?
Read a lot of Forbes posts. Especially the popular ones. No one style is right, but you’ll get an idea of what works on Forbes and what doesn’t. Then try writing a few articles of the type you would post on Forbes, if given the opportunity.