The C-TEK brand has a huge reputation for performance and safety. I've personally had such a charger for more than 20 years which has been in use almost every single day of that time and still works perfectly. They're more expensive than some chargers, but the inbuilt protections against incorrect connection and overheating are worth the price, not to mention that longevity. Some mechanics recommend connecting the two leads from the charger directly to the battery terminals, but conventional wisdom says that connecting the red cable to the positive terminal first and then the black cable to an earth somewhere in the engine bay is a better idea. This method means less chance of a spark igniting gasses emitted by the battery as it charges and discharges. The indicator lights on the C-TEK unit will tell you when you've achieved a good connection. Using a metal part of the car for the negative attachment point works because the whole car is essentially one big negative terminal. Just make sure your attachment point isn't painted or coated with a chemical that prevents the flow of current. Or that it's not a plastic part (increasingly common these days). Many cars have a dedicated post or connection point within the engine bay.