Here is a video tutorial that includes all the detailed steps to make a Twin Loop sling.

Pretty easy sling to make overall…..but it is a bit finicky as you do need to keep things centered and uniform.

This video shows how to do all that centering and uniformity in a simple and easy way.

https://youtu.be/bczvg6p8ULE?is=9e_diET2yqUohzRL

My opinion is accuracy comes from:

1) being consistent (footwork and also sling motion)
2) having a solid base (stepping into the throw while being in control and balanced)
3) releasing the sling with a “normal” (natural) throwing motion

Kind of looks like you got 1) and 2) already. But my opinion, you need to work on 3) most.

Here is what I suggest: 

A) I kind of think slinging is “normal/natural” throwing….just with a different windup. I would say your windup is dominating your motion…..and preventing you having a “normal/natural” throw when you release.  I would suggest video yourself throwing the ball without the sling…..see if it matches how you throw WITH the sling.

B) Also I think if you slow down it would help possibly too….at least until you get more accurate.

C) Finally, I recommend review these videos and see if they help….they did help me a lot.

        https://youtube.com/shorts/vVywF9ha5LM?is=Rw17dbX6OCWDwRNa

        https://www.youtube.com/watch?v=u4-LpLtrGqs

Please update how you compare throwing by hand vs with the sling.  My thought is there will be a big difference.  Curious if that is the case.

Also B) slow down I think will help you figure things out as well. Being smooth lets the sling do the work and generally helps distance and accuracy. Here is video of me slinging slowly…..and you can really watch the sling motion to see if things stay smooth and also see areas that can improve (for example: I swing my entire arm helicopter - would be more efficient to just spin the sling with wrist only)

https://youtube.com/shorts/7TK2pQ624k8?is=EUZ4uu2KEkx0HkGc