You need a thermometer or can do the water method for checking for firm ball stage. To do that, spoon some of the mixture into a bowl of water, at 245-250 degrees it will harden into a consistency that you can form it into a ball that has squish to it but holds the shape if not smashed. To know the caramel is at 350 degrees, you need to watch for it to turn to a medium/dark amber color. Do not burn it. It will make the change pretty quick so you need to watch the mixture more carefully than if you have a thermometer. If using a thermometer make sure it is accurate (place in a glass of ice water and it should read 32 degrees F). Then make sure the probe is submerged into the caramel mixture for accurate read.

For candies: line a pan that will support the thickness you want your candies to be (8x8in works well). Line it with with parchment including the sides, you may also want to spray the parchment with nonstick spray. Caramel is sticky. Shocking, I know. For other uses: if pouring into a batter or on top of another item for candy making, you will want to let it cool and thicken a bit first, checking to make sure it doesnt get too hard. You can always warm it up to thin it out.

• In a medium pot combing sugar, water and cream of tarter and turn on stove to a low/medium heat. You are dissolving the sugar into the water, this part requires you to pretty continuously whisk/stir the mixture. You don't want to get rushed and try to speed it up because that is when you risk sugar crystals forming. If they do form on the side of the pot, wipe them down with a wet pastry brush or CAREFUlly with a wet paper towel. This can take up to 10 minutes, so be patient.
• Once it has disolved, go ahead and place your thermometer into the pot. Turn the heat on medium/medium high. You will continue to stir/whisk until it starts to simmer. Then stop whisking and let it boil until it reaches 350 degrees or it turns amber brown. During this stage- watch for sugar crystals forming and wipe them down if they do. Watch for burning, if it starts to burn BEFORE it reaches 350 degrees (around edges of pot) then gently swirl the mixture and lower the heat. We are making authentic artisan caramel. This means we aren't using corn syrup to help us along. Which means this might take 10-20min. Don't rush it, enjoy the fact that you are making candy the way its been made for an incredibly long time.
• Once it has reached 350 degrees. Turn off the heat/remove from heat. You then are adding in the cream and butter- the mixture will spit at you so be careful- do not slam dunk the butter in there and spalsh in cream. What I like to do is add butter, stir that in until smooth and then add cream, stir until smooth. MAKE SURE YOUR ARE USING A CLEAN AND DRY STIRRING UTENSIL. Silicone spatulas work great and are easiest to clean.
• Back to candy science. Our mixture has cooled below candy making. It's a caramel sauce and that isn't a bad thing BUT it won't get you chewy caramels. To do that, we must heat it back to firm ball stage- 245-250 degrees. So let's get the pot back on the burner and let it boil back up to that temp- it doesn't take long so don't go to the bathroom during this time.
• Once it is at the correct temp, stir in salt and vanilla (or flavor of choice). If you are adding other fun things to the candy- now is the time. Pour the mixture into the prepared pan. Knock the pan on the counter to remove any air bubbles. Sprinkle with extra salt if you like. Then let cool and harden. I will pop mine into the fridge to cool. Or let sit out for a few hours or overnight. Cut caramels into desired shape or eat it all just as it is. No judgment. Caramels will keep at room temp in a sealed container for about 2 weeks. But use your own brain- if it smells or looks off, toss.