Gauge Swatch: US3: 4.25” per 33sts in stranded colorwork
US4: 4.75” Per 33 sts in stranded colorwork.

LTCO 33 sts, turn and knit first WS row in cream so purl bumps show on public side. CastOn with extra long tail to use when crocheting the edge in cream.
On 2nd row (RS) join background color in purple held in right hand. Join with extra long tail to use later when seaming up the sides of the heel.

Tip to Make SSK Decreases tidier and better match to the K2tog Decreases in the Toe Area
On the row before the Decrease row, wrap opposite way the two sts that that will be used for the SSK. The stitches will have reverse mount and can be knitted through the back loop on the Decrease round. This allows you to skip the slipping the two sts that often cause the loops to enlarge. Or just work on the needle tips and be careful to not stretch the loops when slipping.

Backwards Loop Increases
a.k.a. E. Zimmermann M1 Increases R & L:
http://www.craftsfromthecwtch.co.uk/2015/11/a-perfectly-e...
http://knittingnuances.com/resources/Tutorials/Make1Loop.pdf

Remember on the next row when working the increased sts, to work the right leaning backwards loop increase through the back loop or the increased stitch comes undone: You can recognize the backwards loop increase st that needs to be worked through the back loop by noticing the loop is reverse mounted. http://chezlizzie.blogspot.com/2013/02/the-backward-loop-...

CastOn 33 sts for instep
Hold the two strands like LTCO with color for stitch over index finger and rotate thumb strand over top of finger strand each time when color changes.
https://youtu.be/J_CyEumf-rA

Backstitch to Seam the Heel
https://www.interweave.com/article/knitting/backstitch-seam/

Single Crochet Edge—How to start and finish
https://www.dontbesuchasquare.com/add-a-crochet-edge-to-k...

Finished dimensions:
Knitted heel flat: 26 rows to turning row = 3.675” heel ht
Top of slipper length 6.25”
Slipper bottom length=9.5”
Slipper width= 4.375”