I've been making these for years. In all sizes, felted and not.
All you need is a sock pattern that is worked from the top down. Or if you'd prefer felted slippers, find an appropriate pattern for that. When you reach the toe shaping section, use this sequence instead.
Put half the stitches on one double pointed needle which we'll call needle A. Put the remaining stitches on another double pointed needle. We'll call this one needle B. (We will be knitting in the round on these 2 needles.)
Knit across stitches on needle A.
Slip first stitch purl-wise on needle B. Wrap yarn between slipped stitch and the rest of the stitches on needle B. Then replace slipped stitch back on needle B.
Turn work and purl back on needle A.
When you reach the end, do the same thing on this end. Slip one stitch off this end of needle B, place the yarn in between the slipped stitch and the rest of the stitches on needle B. Slip the stitch back on needle B and change direction again by knitting until two stitches remain on needle A. We will now do a ssk. Slip these 2 stitches knit-wise onto right needle, then slide the left hand needle into the front of both stitches and knit them together.
Now, we are going to knit on needle B. The first 2 stitches are knitted together as one. That's called K2tog. We will continue to the last 2 stitches of needle B and execute another ssk.
Moving on to first 2 stitches on needle A, we K2tog and continue to the end of needle A. It's time to slip the stitch off the end of needle B and place the yarn between the stitches. Slip the slipped stitch back on needle B and purl back to the end of needle A. Now, we're going to slip the end stitch on needle B again and wrap the yarn between stitches. Put slipped stitch back and turn work.
Starting from here, On needle A knit to last to stitches, ssk, On needle B, K2tog knit to last 2 stitches and ssk. K2tog on needle A and knit to end. Repeat the slipped edge stitch procedure on needle B. Turn and purl back on needle A. Repeat slipped edge stitch procedure.*
Continue from * to * (above) until there are only 4 stitches left. Work 3 or 4 rounds on these 4 stitches. Break yarn and thread the end through stitches and remove from needles. Pull tight.
Weave in loose ends.
This is exactly the same procedure I described in https://steemit.com/hobbies/@kimmac/how-to-knit-a-gnome
And there are photos there to help. It might be a good idea to try a gnome first to get the hang of the process.
Adults love these too! Have fun and if you have any questions, put them in the comments section!