Step 1: Supplies and Set Up
You will need a woven fabric, I suggest linen or old denim as a beginners sampler to learn how, thread to match or contrast, a needle, an embroidery hoop, and small sharp sewing scissors, a sharpie or other marker to transfer your pattern to the fabric.
Place the fabric in the embroidery hoop, face up, and pull tight, keeping grain(the way the threads run) of the fabric as straight as possible
Use the marker to draw the patten on the back side of fabric.
Step 2: The First Cut
Use the needle to fray the threads in the direction of the pattern. When one thread is long enough to grasp, pull on it until it 1) breaks, or 2) pull out enough thread to reach the end of the pattern piece you are working on. If 1) try again on the next thread,or prick out the first thread a couple of cross threads further on. If 2) snip thread and use that as a starting point to cut the opposite end of the pattern piece.
Pull the remaining threads out one at a time, or fray them up long enough to pull,on either or both sides, until you have removed all the threads within your first pattern area, leaving a rectangle ( or other simple shape) connected to the rest of the fabric on two sides by the remaining threads,and cut free on the other two sides
Step 3: A Stitch in Time
I did both a warp and a woof sample in denim, so the pictures may jump around a bit.
Step 4: Pattern Stitches 101
The stitches can take several forms, each of which looks different.
The first stitch is a basic double cross stitch: Bring the needle from the back of the fabric to the front at the first of the threads you want in this group, bring the needle between the last of the threads for this group and the first in the next group, and pass it across the threads in this group at the back then back forward at the beginning of the group and back through the fabric crossing over the start of the stitch and taking a stitch backwards in the fabric to start the next loop. Repeat across. See the photos of the long bar with the white denim threads .
Basic loop stitch: bring needle up from the back of the fabric near the middle of the group of threads to be separated, pass it behind the group, then back into the fabric as close as possible to where it came out. See first row of small square with the blue threads showing.
Single cross stitch: like the double cross stitch, but keep the stitches shorter so you only cross once under each separate group. See second side of long rectangle with white threads.
Bar stitch: this is basically the loop stitch done backwards, so there is a loop around the threads from back, around the front and back to the back of the fabric, with a tiny stitch to get the needle in place for the next stitch. See last picture with long rectangle.
The "Y" stitch: a single cross stitch with one branch shorter so the thread forms a Y shape. See small square in last picture.
**note: make sure you give yourself enough room to stitch on both sides when the pattern calls for two boxes in tandem in any direction!
Step 5: Grouping Patterns 101
As you proceed down the last side, if you choose to keep the same threads in the same group you will get a bar pattern. If you split the treads, starting with a short group of only 2 or 3 threads, then for the next group use the remaining threads from the first group and half the threads from the next group, you will get a v pattern, which can also make M or W patterns.
Step 6: Grouping Patterns 102: Squares
Starting in the center of a cut end, bring the needle around a grouping of threads from the front of the fabric then pass it around back, and out again at the front. Then run the needle around the first bit of thread from the needle, making a slip knot of sorts. Continue through the rest of square in the same fashion, a series of slip knots, keep the tension of the thread even as you go. This can be done to create bars or v patterns in the open work.
Variations on this include skipping groups of thread in the square and pulling the surrounding groups tight so they overlap (see picture).