Step 1: Step 1- Water
In the morning as you are getting ready to go about your day, head to the nearest sink, bathtub, shower, etc and fill up your boots with water. Fill it up as you would a cup and then dump it out. The point is to get the boot soaking wet. If your boot has drainage holes, it may not fill up but just make sure its completely drenched.
Next, put on two pairs of socks. Having two pairs helps prevent blisters and acts like a moisture reservoir to keep the boots wet all day.
Finally, lace up and go about your normal business. Just wear them around all day as you would normal shoes. The water may feel a bit strange at first but it's not that bad. You may want to re-tie about an hour later or so because the leather will start getting loose quickly and it helps break them in faster if they are tied tightly.
When you get home, open up the laces, pull out the insoles, and throw them in front of a fan to dry. Gore-Tex boots tend to take a lot longer to dry than regular boots but they will still probably dry overnight in front of a fan.
Depending on your type of boot, you may want to wear them wet a couple of times. You should be able to tell a difference in flexibility and stiffness from the new boots before moving to the next step.
Step 2: Step 2- Leather Condtitioner
Some conditioners come in a spray bottle but the spray Glovolium was $3 more so I didn't get it. Use a paper towel or a rag and apply the conditioner to all the outside leather surfaces. If you have the spray, just spray it directly on the boot. The leather will turn a slightly darker color but it's not noticeable from more than a few feet away. The conditioner will take a few hours to dry and depending on how stiff the boots still are after step 1, you may want to add a second coat.
Step 3: Finished (kinda)
Make sure to continue to wear them around after you have applied the leather conditioner. This helps to drive the conditioner farther in to the leather.
The method I have described is much faster than just wearing the boots around dry but it still may not break them to the point they need to be. If you have a 20 mile road march next week, it may be unreasonable to expect to break a brand new pair of boots so that they're soft enough in that time frame. Give yourself adequate time for the task or you could end up with some bad blisters.
Take everything I've said with a grain of salt and tweak it to fit your situation. I've broken in 5 pairs of boots this way and never had any issues but that doesn't mean yours will go flawlessly. Take care and have fun with your new boots.