I'm not sure what a double french braid is. Do you mean making two frech braids, one right next to the other? Or do you mean the fishtail braid?
In case you mean the fishtail. Here are some instructions:
Fishtail Braids
This is a stunning two-strand braid and is a great way to learn the concept of French braiding before tackling three strands.
1. Divide a small section of hair where you want the braid to start (usually at the hairline at the front of the head or just behind your bangs if you have them) into two even sections.
2. Comb each section smooth.
3. Cross the right hand section over the left section.
4. Pull the sections away from each other to tighten the plait.
5. Using the pointed end of a rat tail comb, lift a small section of hair on the right by drawing a line against the scalp from the hairline to the middle of the braid about a half inch to an inch beneath the first plait, depending on the thickness of your hair and how woven you want it to look (more strands create a more woven appearance).
6. Add the new section of hair to the existing section on the right and then cross it over the left section; tighten the plait.
7. Again using the handle of a rat tail comb, lift a small section of hair on the left by drawing a line against the scalp from the hairline to the middle of the braid even with the section you just lifted from the right, add this hair to the existing section on the left and then cross it over the right section.
8. Continue the process alternating right and left sections, tightening each plait as you make it, until you reach the hairline at the nape of the neck.
9. Secure the remaining hair with a hair-safe ponytail holder. You can then let the remaining hair hang free or weave a standard English braid to the ends.
Tips: For a different look, cross the sections of hair under each other instead of over. Keeping your hands close to the head will create a tighter braid, holding your hands away from the head will create a looser, more drapey braid.
Here some instructions for the french or dutch braid:
French Braids and Dutch Braids
This is a beautiful braid that looks much more complicated than it is. It can be done in two styles, the overbraid which creates the classic woven appearance of a French braid, or the Dutch Braid (sometimes called an underbraid or inverted braid) which creates a braided rope that sits on top of the hair.
1. Divide a small section of hair where you want the braid to start (usually at the crown of the head) into three even sections.
2. Comb each section smooth.
3. Cross the right hand section over the middle section. The original right hand section is now the middle section.
4. Pull all the sections away from each other to tighten the plait.
5. Cross the left hand section over the middle section. The original left hand section is now the middle section.
6. Tighten the plait again.
7. Using the pointed end of a rat tail comb, lift a small section of hair on the right by drawing a line against the scalp from the hairline to the middle of the braid about a half inch to an inch beneath the first plait, depending on the thickness of your hair and how woven you want it to look (more strands create a more woven appearance).
8. Add the new section of hair to the existing section on the right and then cross it over the middle section; tighten the plait.
9. Again using the handle of a rat tail comb, lift a small section of hair on the left by drawing a line against the scalp from the hairline to the middle of the braid even with the section you just lifted from the right, add this hair to the existing section on the left and then cross it over the middle section.
10. Continue the process alternating right and left sections over the middle section, tightening each plait as you make it, until you reach the hairline. Secure the end of the braid with a hair-safe ponytail holder or continue weaving an English braid to the ends of the hair.
Tips: To create a Dutch braid, cross the sections of hair under the middle section with each pass instead of crossing over it. Keeping your hands close to the head will create a tighter braid, holding your hands away from the head will create a looser, more drapey braid. This style can look stunning with the braid ending at the hairline and remaining hair caught in a ponytail, with the remainder of the hair being braided, or with the remaining hair tucked under the woven sections.