This is the provisional cast on I use.
Here is another video-
With the above linked crochet cast-ons I use waste yarn and a smooth (not
fuzzy) yarn in a slightly lighter weight if possible. You just start knitting after the cast on. For stockinette you knit a row and then purl a row if you're knitting flat or knit all rows if you're knitting in the round. Keep track of the number of rows.
When you get to the fold of the hem you can do a turning row so it lays nicely flat. If you're knitting stockinette you can get a simple turning row by purling one row. I twist my purls to make them tighter, but it's not necessary though.
After the turning row you just knit the same number of rows you did before you did the turning row. When you have the same number stop.
The easiest way to join IMO is to put the stitches from the cast on row on a spare smaller needle. Then you hold the two needles together and knit one from each needle together like you do with a 3 needle bind off. Scroll down on this page to see that -
This video also shows a turning row and how to join two edges for a hem.
I know you've already done your hem now, but next time give this a try.