Try this as suggested above.
Put your arm through the sling and push your elbow out to pull tension on it.
Keep in mind that the standing position is the shakiest and will take lots of time and practice to become proficient in. This practice is well worth it.
If you have a tennis ball lying around, try practicing on that, at about 10m. Once you can hit it consistently, move it out to 15m and so on.
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