This achievement will unlock when your current total materials are greater than or equal to 1 million. You can see how many materials you have at the top right of the menus when your are at your shelter. I would recommend playing normally until you have reached max shelter and built level 1 of each improvement. A large part of the million will end up coming for resources so getting those two achievements done first will be easier in my opinion. Now we will get into each way to earn materials.
Passive earning: While leveling up your shelter you will notice that the Wood Log and Box of Herbs will earn you materials per hour. These improvements should always be upgraded if you have the resources as it will help speed up the process. I had Wood Log at level 7 and all Box of Herbs at level 10 by the time I unlocked this achievement. This would earn me ~9.5K just for logging in and doing my daily challenges. You should also get the Chemical Distillation improvement up to level 4-6 to increase your gain from deconstruction. Anything past level 6 will start to have negative returns as it costs more than would be gained.
Crates: You can earn crates through daily challenges or by getting the airdrop at the end of each match. Make sure you either complete your daily challenges or abandon them before they reset because if you don't you won't get a new challenge for that slot. These rewards can really make a big dent in the overall total depending on the rarity. The crates can vary in value but will usually in the ballpark of 1-30K worth of materials after deconstruction. If you want to read more on what I have to say about crates check this guide. Solution for Gun Freak in Vigor
EDIT: You can now earn crates through the season pass just by playing regularly. Also you can earn crates by donating food at you shelter every 28 days.
Deconstruction: I will go into some detail about each section and have sorted them by most to least valuable.
Resources- This is where the majority of your materials will come from. As i mentioned before you should probably upgrade each improvement once but beyond that I would avoid upgrading anything that requires purple materials and be conservative with your Chemicals. Electronics and Fuel are worth 200 a piece and Chemicals are worth 50 a piece so these are the real important ones. Just to make a point, upgrading the Electric Generator from level 1 to level 2 will waste about 170K worth of materials.
Weapons- I recommend finding a few guns that you are comfortable with and sticking with them. I personally rotated between using the Thompson, Grease Gun or the Soumi KP/-31 exclusively. Remember its up you which guns you use but using more expensive guns will make the process take longer. Also melee weapons, pistols and shotguns just aren't good enough to deal with long range encounters or multiple enemies so I avoided them. I sold all of my guns that weren't the three I used to knock out around 300K materials.
Weapon Parts- You can either turn these into the guns for a free magazine of ammo or just deconstruct them in bulk. If a weapon takes 4 parts to craft then the part will be worth exactly 25% of the gun.
Ammo- Ammo costs the exact same amount to craft as you get in return for deconstructing them, so don't stress about anything here. Just deconstruct everything as you are nearing the 1 million milestone and you can buy it right back.
Consumables- Depending on whether you use these on your load out or not will determine how many you have on hand. However, there typically won't be a large amount to gain here anyway.
Consumable Parts- Not currently in the game and even if added they wouldn't be worth much assuming they work like weapon parts.
Finally here are a few looting tips:
Because of the potential rewards, be prepared to go after any crate that is blue or higher.
Always make room for purple and blue resources.
If you have the chance, take whatever gun is most valuable. Don't take someones shotgun over a high quality alternative. Always take machine pistols over regular pistols and knives.
Destroy or avoid picking up handgun and knife weapon parts. These are worth next to nothing and take up precious backpack space.
Ammo can also be destroyed if you need extra room as they aren't worth much.