The first things you should know about reviewing on Amazon is that you should read
Amazon TOS and follow their guidelines.
A lot of websites, groups and seller will ask about your Amazon Profile link. The way to find your link is go to the your amazon.com located near the top of the page. Click on your profile link at the top of the page. Copy your url until you get to the ? mark. That is your Profile Link. This link take you to all the review you have done on Amazon. DO NOT Post this link publicly. They are several people in the world that will do and down vote your reviews just because they feel like it.
You can also find your Ranking on your profile page. This number is based on how many helpful votes you receive. So it is going to go up and down based on your reviews and others.
A good review should be at least 70-75 words before your disclaimer. Uploading pictures and videos will improve the quality of your review too. I know I use good review with picture to help me make decision on buying items. Explain way you like the item and why you don't. Seller feedback is also a thing to do. That is where you should post any advice on improving the product.
ALWAYS add a disclaimer to any review that you got the product at a discounted price or for free.
Example disclaimer is "I received the product at a discounted price in exchange for a honest and unbiased review." Or "I received this product for free in exchange for a honest and unbiased review." DO NOT ask for anyone to mark you review helpful. This will get your reviewing privileges banned. DO NOT included where you received the discount either. Since there are several people out there that target reviewers to down vote your review. I suggest leaving a simple and small disclaimer.
DO NOT post the review for the first 2 days of having the item. Most reviewers I know count the day you receive the product as day 1. Supplements should be reviewed at least a week after you start testing it.
Amazon has been having some issues when posting reviews. It is best to submit just a written review then go back into the review after the review go live to edit the review to include pictures or videos.
As a reviewer you should no longer up or down vote any reviews. This can flag you account and cause you to lose your reviewing privileges on Amazon.
Come up with a way to organize you reviews. A lot of reviewers use spreadsheets to keep track. I keep telling myself that it a more effective way to organize but haven't started using spreadsheets yet. I have a odd way of keeping take. I use my phone for a lot of what I do so when I started, I would simple write done in my memos the product I ordered right after I placed the order. Then I would use emails and Facebook messanger to keep track of where the review goes. Not effective, I know.
UPDATE:
I forgot to include do not share codes. You CAN NOT sell anything you get.