Easy decrease patterns

I found a pattern that is a multiple of 10 + 1 - I would like to use this pattern to make a mermaid tail blanket, (sack) however I don’t know how I would go about decreasing this even if I added extra stitches at the beginning and just had the actual pattern in the middle. Can anyone do a bit of a rewrite or give some direction as to what to do to make this work. TIA! https://www.pinterest.ca/pin/143481938113801988/

That’s a lovely pattern for a fishtail. Why not use a pattern like the one you found earlier and substitute this stitch pattern for most of the body? When you get to the lower body decreases, change to stockinette stitch since the rapid decreases in most fishtails would disrupt this pattern.
Knitting a swatch will help you determine the size you get for the stitch pattern and then calculate the number of sts to cast on.