Modern sewers are used to seeing entire pattern panels on their tissue patterns. If a skirt has four gores then we are used to seeing four gores in their entirety (or two which we cut twice) which we then lay on the fabric. We are also used to fabrics of 150cm (60") wide which many patterns are catered for, rarely do they exceed this.
In Alcega's day tissue patterns were not available and tailors used unscaled diagrams and a yard stick and chalk/soap/ink to draft the patterns on the fabric. Modern tailors too still use measuring tapes and diagram books rather than patterns. And there are directions throughout the written history of tailoring and dress making of taking patterns from existing garments.
These tailoring books were meant to illustrate how to cut the pieces of a garment to take best advantage of the fabric which was quite narrow for silks though less narrow for plain wools. As such many larger panels overlapped the edges of the fabric and had to be cut along those lines and the extra pieced cut out elsewhere.
Tailors were allowed to keep the excess cloth and when you deal with cloth of gold or silver (made with the real metals) you can see how precious these scraps could be. Clients however purchased the cloth from other merchants so would be reluctant to buy more cloth than absolutely necessary.
Generally the panels are worked from the bottom of the fabric, the CF of the skirt is laid down first and then the CB with extra pieces being cut to extend what will generally be the bottom 1/3 of the side seams. These extensions are unobtrusive as they sit to the sides of the skirt and near the hem. The piecing also seams parallel to the front and back of the skirt and usually with the selvage.
In the above example the skirt front is on the left of the pattern (note it has been rotated 90 degrees CCW for the purposes of this illustration) and is cut first, the back next and then the extensions at the side cut from the remaining.
Without understanding what most of the symbols mean it is still possible to work out what part is what by remembering the back is generally cut fuller than the front. The symbols represent both measurements and piecing. I ignore the measurements to a degree and use a proportional method to account for my height.
Note also how the piecing does not match perfectly, this is simply due to the nature of the diagrams, they are supposed to match when you make your skirt!
Some patterns suggest marking the back skirt panels first. This is easier for gowns without a long train than for those with! A train can easily be cut shorter if needed if the skirt front is marked out first. it is always harder to try and find extra fabric for the front of a skirt which cannot be adjusted.
Here is a rather more complicated diagram. Following the letters, which indicate where pieces match together, it is possible to fairly easily come out with the final shape.
However we do need to refer back to either the original text or to the translation to realise where the fold lines are and where the CF and CB are.
I would recommend using pencil and paper and tracing the diagrams and assembling them as exercises in familiarising yourself with how the full sized panels should look. The piecing lines will fall in different places depending on the width of fabric used.
In some diagrams the fabric is on the fold and in others it is full width. These fabric widths are also different to modern fabric widths! It is the full panel that will determine the fall of the fabric nor the small extensions. These are incidental and aside from adding a little extra bulk with the seam allowances will not affect the fall of fabric.
Over time you will learn to be able to look a piece of fabric and mentally determine the best use of the fabric. Another fairly simple way would be to copy my half circle skirt diagrams and do the same as they are less complicated.
I have chosen the simplest gown style with a train to show how to easily turn the diagrams into full scale patterns.
We first decifer which pieces are which as this gown has a doublet bodice (jubon), half circle shaped sleeves (manga redonda) and a skirt with train (saya).
Once the above pieces have been identified the first place to start is with the skirt front. The measurement from waist to hem will be the guide to scale all other lines of the gown. This is not an arbitrary choice but the one that allows for the easiest scaling of all parts.
It is the angle of the side seams that most determine the shape of the skirt. So by extending the lines of the side seam and the CF seam we wind up with them meeting at some point above the panel. This gives us the angle. It is up to you whether you make note of it and use a protractor or if you simply make a paper copy of this angle to place on the fabric as a guide for the tape measure.
When scaling it is percentages that matter and so we now have the ability to determine where the waist is to be cut. The length of the distance from point to waist can be expressed as a percentage from the hem to waist. (Point to waist/ hem to waist).
I have already calculated the percentages for this pattern which you are free to use for your own scaling experiments).
Generally it is best to cut skirts with the hem at the bottom end of the fabric. This allows for the curve of the hem and waist to be adapted on the fabric more easily.
Here I have used my approximate height to determine the final dimensions of this skirt piece. This leaves me with the waist being 22cm on this panel, 44cm on the full front. This allows for some ease or to wrap the seam further to the back than true side- as can be seen in the majority of skirts from this period.
Using the above guide I can now see where my piecing will lie on modern fabric that is on the fold. Were I to cut these from fabric opened there would be no piecing required.
By using the same principles of extending the side and CB seams to a point it is possible to also scale up the skirt back panel again for fabric on the fold.
First match up the piecing to find the true final panel shape. Match the length of the side of the front piece for the side of the back piece as it will by necessity be the same length and then the train length will be determined and adjusted according to taste or need.
Due to the nature of the diagrams it is not possible to scale up and have a perfect pattern! For one you would need to have the same proportions as the digram and for another the diagrams where drawn and engraved with care but were a guide only.
For myself I increased the flare (wider angles) and added a little extra at CF and CB so the front would sit smoothly over the top of my hips in my c 1560s Spanish Gown. I also shortened the train.
For my Italian gown of red velveteen I increased the flare but a little and increased the waist at front and back to allow for pleating. I did this by laying my Valois skirt on the velveteen but set it in by about 10" from the selvage- giving me 20" extra in the back. I did the same at the front.