Tip1: Someone once showed me once that if you are in question as to the appropriate range of needle for your yarn (like, it's homespun, or you lost the ball band), you can fold the yarn in half and poke it through the holes of a needle sizer. The hole that the yarn fits through most comfortably is a good size for your needle.
Step 1: Make a gauge swatch. This is critical! Measure the number of stitches it takes to reach 4in (10cm). The number of rows isn't particularly important. Divide this number by 4.
size 8 needle, worsted weight, 18 st per 4 in
Step 2: Multiply the number of stitches per 4in (10cm) by 13 . Adjust this number up or down slightly to make a number that is divisible by 6.
= 52 becomes 54
Step 3: Cast on the number that you created in Step 2. Join, being careful not to twist, place marker, and start working your pattern.
Tip2: Note that this formula will work for lots of types of stitch patterns, including ribbings. Remember that you'll be flipping up the cuff, so it's good to choose a pattern that is reversible. With the example in the photo the yarn was so fuzzy that I just did stockinette stitch; I've also done this in adult hats with a complex cabling pattern that morphed into simple ribbing. It is of course easiest if you are doing a stitch pattern that is itself a multiple of 6, but you can also fudge things later. Example: If your stitch pattern is a multiple of 10, and you would cast on 72 stitches for gauge, go ahead and do 70, then when it comes to the crown you can deal with the extra 4 stitches. This is an advanced adaptation of the hat!
Step 4: Work chosen pattern for 5in/12.5cm [7in/17.5cm]
Tip3: If you choose to have an uncuffed hat, work for 4in/10cm [5in/12.5cm].
This hat used the 4 in with a crocheted cuff.
Step 5: Start crown shaping. If you took the Tip2 route and have a number of stitches that is NOT divisible by 6, decrease evenly around the crown to reach a number that is divisible by 6. Place five more markers evenly around hat; you'll have six sections marked.
Step 6: Decreasing follows this pattern: Round 1: K all stitches Round 2: (SSK; K to marker; slip marker) six times (decrease by 6 stitches per round) Repeat these two rows until 6 stitches remain. Cut yarn, leaving a long tail. Using darning needle, thread yarn through stitches that are live on the needles. Slip stitches off needles and cinch closed. Secure yarn carefully to inside.
Tip4: You can modify the crown several ways. I'll be doing some examples of alternate crowns in a few weeks. In the meanwhile, try doing it in purl, instead of knit, or stick to your pattern while you decrease. You could also play with more or fewer decreases per round; increasing to 10 will get a flatter curve while having only 4 will get a steeper curve to the crown.