This pattern is based on “Alpis Pfeilraupe”
I simplified the instructions as follows:

Each row consists of knits, you will create a rib pattern.
In order to create the tapered structure of the shawl you will increase 1 stitch at the beginning of every 8th row beginning at row 9 until the end of the project and 6 stitches at the end of every uneven row to the middle of the project, after which you will decrease every uneven row until the end of the project.
The slits are created in row 17 and then every 16 rows for a total of 6 times
The middle of the project will be at row 9 between slit 3 and 4.

Material
I used two ply handspan yarn, Ashland Bay 70% Merino 30% Silk blend Concord and US 10 circular needles, at least 42 inches or more. Don’t ask me about the amount of yarn used, at this point I hope to have enough material. Edit: nope, I was wrong, at the 55% mark I’m running out, spinning more as I type.
The yarn you chose will determine your needle size and have an impact on the size of the project. Mine is getting rather large, I will post the final measurements when I am halfway through the project. The projected width is 34 inches, length about 70 inches. Nicely warm and cozy.
Slip the first stitch to create a smooth corner, otherwise you might want to crotchet around the project at the end to smooth out any uneven sections.

Instructions
Cast on 12 stitches
Row 1 Knit 11 stitches, place stitch marker, knit last stitch. Cast on six stitches
Row 2 all knits
Row 3 knit 11 stitches, move marker, knit to the end, cast on six stitches and place marker after the 11th knit
Row 4 knit, follow stitch markers (you should have two stitch markers after the 11th and 22nd stitch, this is where you later make the slits)
Row 5 - 8 continue in pattern, increasing at the end of every uneven row on the side moving further away from the stitch markers
Row 9 Add 1 stitch at the beginning of the row, continue in pattern
Row 10 - 16 continue in pattern
Row 17 add 1 stitch at the beginning and make the first slit between the stitch markers by loosely knitting the first and second stitch after the marker, slip the first over the second, knit the third, slip second over third and so on until you slit the last stitch before the second marker over the first after the marker. Knit to the end and add six stitches.
Row 18 knit until you get to the decreases, cast on 11 stitches, place marker and knit to end
Row 19-24 continue in pattern increasing 6 stitches at the end of each uneven row.
Row 25 add one stitch at the beginning and follow pattern as established
Follow pattern until row 10 between slits 3 and 4 where you will decrease 6 stitches at the beginning of each even row. Still increase one stitch at the beginning of each repetition.
At the end of 30 repetitions the shawl is done.