You will want a fairly substantial yarn for this otherwise the scarf will be too flimsy .
Cast on 3sts.
Row 1: K1, M1, K1, M1, K1
Row 2 and all even rows K across
Row 3: K1, M1, K3, M1, K1
Continue in this fashion till scarf width is about 3 to 4 inches (depending upon the intended wearer) having an odd number of stitches across.
Knit in garter stitch (knit each row) for another 5 to 7 rows (I sometimes shape this a bit so it looks like an upside down heart but it isn't necessary)
**this is the tricky part** but trust me it is harder to explain than to do
Next row: K1, P1 across to last stitch K1
Next row: P1, K1 across to last stitch P1
Continue in ribbing pattern for 3 inches, leave these stitches on a spare needle. (I wrap elastics around the ends to keep the stitches from slipping off)
Go back to where you started the ribbing and pick up a stitch in each of the stitches across, repeat the ribbing till the two pieces are equal in length.
Holding the two needles parallel to each other knit the two pieces together by knitting through a stitch from each needle together.
Continue in garter stitch (knitting each row) to desired length. I find about 15" to 18" works well for most people.
Work another 3 inches in ribbing, then work the same amount of rows in garter stitch to match the first end. Decrease down to match the beginning keeping K1 on each side. Cast off when you have 3 stitches left.
Tuck the one end through the ribbed double section and wear.
Lots of variations abound... tassels or pompoms are fun for kids but most men don't seem to appreciate them.
You could knit the triangle tails in stockinette and duplicate stitch the person's initials in etc.
You could also work a cable up the middle of the main section or some sort of patterning.