From 1975 to 1996, the car was parked in my grandfather's brother's garage.
My grandfather moved the car on a trailer in 1996 to his garage a few miles away. It has been standing still there until this year when I rolled it out and started it. This was the first time someone started it up in 38 years!
Remove the spark plugs and squirt some oil in each cylinder and let it sit over night.
Try to turn the motor by hand by rotating the cooling fan. If the pistons are stuck, you can stop now and call for help.
Check if the battery is ok. Otherwise you have to buy a new one. In my case, a 6 volt battery.
Replace all fluids. Brake fluid, coolant and engine oil. Replace the oil filter and fuel filter. Drain the gas tank and the fuel lines.
Check if the coolant and heater hoses are ok. Check if the belts are ok.
If you are planning to drive the car now you need to check if the wheel bearings and brakes are ok.
Now it's time to try to start the car. A good idea is to buy a can of starting fluid.