This is what I would do. Cast on using the magic cast-on
method, casting on the number of stitches that you would have been left with had you knit from the top down and had finished your toe decreases. Then increase 4 stitches per round, every other round, until you have the total number of stitches that had been used for the foot of the sock. Then just carry on and complete the foot.
When you get to the heel....I know there are ways of doing a heel flap for a toe up sock, but I don't really know how cause I'm just learning toe-up myself, but if you do a short row heel
, it doesn't matter if you're working toe up or top down, you do the heel exactly the same way.
Once the heel is done you just work the leg until you get to the cuff, then bind off loosely.