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The absolute last thing you want to do is to injure your arm just when you're starting to practice a new skill. Make sure you do a dynamic warm-up in which you get your arm moving gently first. A few ideas for warm-up exercises you can do are: Swinging your arms horizontally and vertically for 30 seconds each way. Using an elastic exercise band to stretch your throwing arm. Gently moving through the throwing motion 20 times. The direction of the spin should be such that the ball is spinning forward. Doing this isolates the actual action which creates the spin on the ball. You can do this anywhere as long as you have a baseball and you can toss the ball up a few feet in the air. When doing this, really try to emphasize the flick in your fingers as you drag them down over the front of the ball. The goal of this is to maximize the revolutions you can get on the ball but start out by just mastering the direction of the spin and then working towards full revolutions. When you pitch, putting your whole body into each pitch is really important as this gives you the maximum force you can get. However, when you are just starting to practice the curveball, isolating your arm and hand action is helpful, which is easier to do when you're on your knees. This is also a great way to build muscle memory and minimize the number of factors involved in your throwing action. A really important part of throwing a curveball is ensuring that you get the direction of the spin correct and that you can repeat this at least 80% of the time. As such, throwing at half your regular speed is a good halfway point to get your action started but to minimize the variables you have to battle. If you feel like you want to start at lower than half your regular speed then that is absolutely fine, the key is that you don't jump straight into 100% and sacrifice your accuracy. Don't be too concerned if you aren't seeing too much curve from the ball here. Everything you're doing at the moment is working to maximize the efficacy of the throw when you move to 100% effort. After practicing on your knees and getting the hang of throwing a curveball, it's a good idea to try pitching while standing up so you can practice throwing with a fuller motion. This is a more realistic representation of how you will actually be pitching when you play. You don't need to start out at full effort level here, the most important thing is that you are standing up and preparing to use your lower body. This is a skill that takes hours and hours of practice but the results are incredibly rewarding. Always listen to your body if your arm is getting sore and take breaks as necessary. Sometimes videoing yourself pitching can help as you can pick up on errors you are making that you don't realize you are.
Warm up your arm by doing a few different arm exercises. Practice flicking the ball upward while getting spin on it. Get down onto your knees while you're practicing. Throw the ball at half of regular speed to your partner when practicing. Practice pitching standing up once you get more comfortable with the throw. Practice every day if you can!