You'll want to begin the task of gathering pecans as soon as possible after they begin to fall . You can even use a long pole to shake/knock the tree limbs so that the nuts will fall. If they are on the ground too long they will get wet and begin to rot. You could also be in competition with squirrels or other animals who gather nuts.
Plus, it's a good idea to get them off of the ground if the area is prone to holding moisture as rain could easily cause a muddy mess, getting both you and the pecans dirty. If is a good idea to mow the grass beneath the tree fairly low before the harvest. Doing so will help in the gather process. You'll know they're about ready to fall when you see the husks (the tough casing, which starts off green in the summer and browns as the pecans begin to ripen, that surrounds the nut) begin to turn dark brown and start to open, exposing the pecan it had been protecting. We usually have some clean ice cream or 5-gallon buckets around that are handy for dropping them into. Kids love plucking pecans off the ground and throwing them into a bucket. This is a good job for little ones as mom or dad uses a pecan harvester or pecan picker upper to capture the nuts (if all else fails, you can just pick them up by hand, the "old fashioned way") and then empty them into the bucket.
Our son, however, really likes them and keeps us pretty busy cracking "cons" for him to eat while we're out there. After the pecans are all rounded up (may require collecting multiple times) you'll want to store them in a dry location or crack them soon to get the "meat" out. They can be stored in the shell in a cool, dry place for over 12 months. Just make sure they are dry before you store them. You may have to spread them out on some newspapers to dry if they are damp. Let them dry for a few days and then put them back into a container, preferably one that will allow them to "breathe" (feed sack or old pillow case), for long term storage. Don't forget to protect them from rodents. If you are going to crack the pecans right after harvesting them you can do it yourself with a nut cracker or have someone with cracking equipment do it for you.
Ethan with weird pecan he harvested!