You're in a relationship
Plain and simple, an endorsement deal is a partnership where both parties do specific things to help themselves and one another. So at its best, an endorsement relationship is a win for all involved. Which means you should...

Only endorse a company whose products you love
Companies don’t want musicians looking to chalk up another endorsement, They’re looking for advocates who love and use their products, people who will go out and proselytize for their brands.” Which means you should only endorse a product you use regularly, know well, and genuinely like.

Audiences can sense insincerity in a heartbeat, and endorsing a product for the wrong reasons will destroy your artistic credibility, you don't want your listeners distracted by any lack of integrity. You want them focused on your performance! So in an endorsement relationship, make certain your priorities are aligned. The alignment of priorities means we're naturally helping one another.”

The timing has to be right
When you distill it all down, the ultimate purpose of an endorsement is to help grow a brand, says An endorsement deal is not an artist getting a product without responsibilities in return. If the artist doesn’t qualify to help grow the brand, then the manufacturer wants that non-qualifying musician to purchase his or her gear from a local dealer or favourite Internet dealer.

It’s more important to build your career and fan base than to spend energy at the beginning of your career seeking an endorsement. If someone is a brilliant songwriter or player but he doesn’t play any live shows and only has 200 views on YouTube, he’s not going to help a company sell their product. Company's need to have a compelling reason to align with brand.”

Be positive and professional
If you feel like your band is in position to get an endorsement relationship rolling, The next step is to reach out to the company and ask for somebody in artist relations, The dialog can begin from there.

If you attend NAMM, AES, or other music industry trade shows, introducing yourself at various manufacturer's booths never hurts, even if you won’t be ready to approach the manufacturer of your dreams for another several years. Having made a connection within the organization can be a great asset when it comes time to talk business. Landing and maintaining a strong endorsement relationship has a lot to do with positive energy, good will, and enthusiasm. In other words, be positive and professional.

Negotiate
There are all flavors of endorsement deals, so be sure to approach the negotiation with a full understanding of what you are and aren't agreeing to. “In addition to using the manufacturer’s goods, an artist will generally be responsible for providing a photo and a quote or some kind of testimonial about the instrument or product. There might be the requirement to include a logo on your website or possibly do an in-store clinic or two, depending on the band.

One point worth careful consideration is any contract that specifies which instruments from competing companies you can and cannot use in public. So you need to study the fine print before signing anything, and be sure whatever you agree to does not limit you or compromise your ability to deliver the best performance and music possible. As with any legal or binding agreement, you should consider showing the document to a lawyer before you sign.

Build your career first
Company's crafts endorsement relationships with plenty of up-and-coming artists, but regardless of whether a band is selling out small clubs or packing stadiums, true dedication to one’s career is the best way to attract endorsers is to build a robust, exciting, and vibrant career., He goes on to say the best way to attract endorsers is to build a robust, exciting, and vibrant career. you should be spending time making connections, handling social media, rehearsing, playing gigs, etc. If you get popular, endorsements will come.