The following are random tips that I hope will be useful to you.
1. Use 100% Wool Worsted weight yarn. I like Lions, Fishermans, Cascade, Ella Rae, Classic Merino Wool, Patons just to name a few.
2. Before you start sewing, make sure that the bottom of the slipper has the "knitted" side out (this is the RS - right side). Also, make sure that the uppers both match and have the RS face up.
3. When felting your slippers, just when you think "this should be good" wash them one more round and then it's usually "yup, that's it - smile"
4. When fitting your slippers during felting, you can pull and stretch and fit the slippers into place on your foot. If the fit is still too loose, continue washing. If it is good, leave on foot for a moment to cool from the hot wash cycle. Slide off foot and air dry. If form does not stay in place, stuff with plastic bags to help hold form.
5. I like to use a nylon laundry bag for felting. If I am felting a large size slipper, I use two bags. One slipper in each bag to give them enough room to felt evenly.
6.When sewing your slipper, make sure that your stitches are loose. While you are sewing, you can pull on each side of the stitch to loosen it a little. Your sewing should mimic your knit tension, this will vary with each persons style of knitting.
7. I like to knit the i-cords two at a time. Knitting the two i-cords at the same time will help keep them the same length and it makes it go by faster. I don't know if this shortens the time any, but it sure makes me feel fast and clever :)
8. Ideas for quicker drying time. Set your slippers over the vents during the winter when the furnace is on or out is the sun when it warm and sunny.
I hope these tips come in handy when you are creating. I enjoy making my slippers and feel proud when I see them finished. I hope you do too. All my best~