How do I put antifreeze in my car?

Loosen the reservoir cap just a little and step back a bit as the pressure releases just in case. Then, remove the cap completely. If the coolant level is low, add the correct coolant to the reservoir (not the radiator itself). You can use diluted coolant by itself, or a 50/50 mixture of concentrated coolant and distilled water. You might want to use a funnel to prevent the fluid from spilling.

How do I check the coolant level?

Instead of taking off the radiator cap, just check to see if the liquid reaches the Full line on the side of the coolant reservoir. If the liquid doesn't reach this line, add a 50/50 mixture of water and coolant until it does.

How often should I check this?

It might seem impractical, but you should really check your coolant/antifreeze at least every couple of weeks, depending on the amount of traveling you do and the age of your vehicle. The good news is, most cars will alert you to problems via a gauge on the dashboard.