Hire a strength and conditioning coach. You will want to ensure that you are in the best shape that you can be in for when you try out. This person will put you through the paces you need in order to compete on the same level as others at the tryouts. You may well be facing players who are either in the NFL or have NFL experience and are trying to hook back on with a team.
Choose which NFL team you would like to play for. If you have no preference, research each team website to see if they post any announcements about open tryouts or mini-tryouts in your area. You will most often find that they do hold camps across the country in the early spring to find new players.
Put together a highlight reel about your prior experience. Perhaps you were a college player, or someone who played in the CFL (Canadian Football League) and want to try for the NFL. Any film footage of your playing days could help you get an invitation to a tryout. If the coaches like what they see on the DVD or CD, then they can contact you and tell you when and where to be for their tryout. There are 32 NFL teams, so visit their websites and find their contact information. Send several copies addressed to the coaching staff to ensure that someone will get it. Include your contact information as well.
Attend an NFL evaluation camp. Contact the league to ask when they have the next one scheduled, and make plans to attend. A majority of the players attending are most likely just like you, looking for an opportunity to be discovered.