You can attach the hair directly into the head of the doll. The way of knotting remains the same, you just leave out the little cap.
But you can also crochet a base (a cap) in the same color as the hair and then knot the hair in it. Crochet a circle first (sc 6 in MR, inc x6, (sc 1, inc) x6, (sc 2, inc) etc. When you reached the needed diameter crochet a few more rows without increases so that the cap fits well the head (not to loosely). The easiest way is to use the information from the pattern for the head and take a half or full hook size larger (if you are using wool of the same weight). Otherwise you just have to try the cap on more often.
For the little cap on the photos I only crocheted one round without increases, so the little cap is quite flat. Usually more of these rows are crochet, it really looks like a little cap. How many you need depends of course especially on the figure.
At the end I leave a long tail, with which the wig is sewn to the head later.
When the cap is ready, you have to cut the threads for the hair. To do this, cut wool to the same length (the individual threads must be twice as long as the hair should be later, as they are taken twice when knotting). The fastest and easiest way to do this is to wrap the wool around an appropriately wide piece of cardboard and cut the wool on one side. The side of the carton you wrap around should be as long as the hair should be later.
With the crochet hook (here you can use a slightly smaller one) pierce the base of the wig, take the thread twice so that a loop forms and pull it through.
Now pull the two loose ends of the thread through the just formed loop and tighten.
In the two outer rounds I always knot one thread in each stitch around. Inwards, so that the hair does not become too voluminous, I leave out rounds and/or stitches. By using the base, you can simply leave larger gaps here, since the scalp cannot shine through. If you do without the little cap and knot directly into the head, you have to knot a little closer.
Now just place the wig on the head and fix it with pins. Sew the wig all around with the end thread that you're leaving before and knot it well. You can just let the thread hang.
Finally you have to cut and style the hair :o)