Sankalpa should be made in a simple language, which your subconscious understands well. It should be free from the double interpretation and stated in a short and positive way as if you were explaining it to a child. Each time, choose only one intention for the practice, and possibly, repeat it for the next few practices until it comes true. You should prepare for your resolution to take some time – depending on how complex it is it could either come true the same day or in a few months, or longer.
The essence of Sankalpa should be a change in your life, which is in your power, that can be attributed to your efforts. You shouldn’t wish something out of your control, such as “I can time-travel” (well, unless you are a scientist working on time travel machine that you believe has a potential to succeed).