How to start my 07 gmc yukon denali when it's cold outside and it won't start

There are a few things you can try to troubleshoot the issue:
Check the battery: Make sure the battery is charged and in good condition. Cold temperatures can cause the battery to lose power, so it may not have enough juice to start the engine. If the battery is weak or dead, try jump-starting the vehicle.
Check the fuel: Make sure there is enough fuel in the tank to start the engine. If the fuel level is low, add more fuel to the tank.
Check the spark plugs: Cold temperatures can cause the spark plugs to malfunction. Remove the spark plugs and inspect them for damage or wear. If the spark plugs are dirty or damaged, replace them.
Check the fuel pump: If the fuel pump is not working properly, it can prevent the engine from starting. If you suspect the fuel pump is the issue, have a mechanic diagnose and repair it.
Check the starter: The starter is responsible for turning the engine over when you turn the key in the ignition. If the starter is not functioning properly, the engine may not start. Have a mechanic diagnose and repair the starter if you suspect it is the issue.
Check the alternator: The alternator is responsible for charging the battery and keeping the electrical system running. If the alternator is not working properly, the engine may not start. Have a mechanic diagnose and repair the alternator if you suspect it is the issue.
If none of these troubleshooting steps help, it is possible that there is a deeper issue with the vehicle.