Own work. Schedule #2 of Guyana's Constitution initially included a crude black and white image and description of the flag.

Scans of Schedule #2 (from an old printed copy of the constitution) are visible in: 2001 Bill 7 2001 and Act 6 2001

Vert upon a pile throughout issuant from the dexter or, fimbriated argent; white upon a pile gules fimbriated sable, upon the same base.

The design and the colour of Guyana Flag are interpreted as follows - The green background symbolises the agricultural and forested nature of Guyana. The white symbolises its waters and rivers potential. The golden arrow symbolises Guyana’s mineral wealth and its forward thrust. The black border the endurance that will sustain the golden arrow’s forward thrust into the future. The red triangle represents the zeal and dynamic nature of the nation building that lies before this young and independent country. The Garter King of Arms has described the design as being on of the most significant departures in national flag design in modern times. The Flag is designed to be of a proportion 3 × 5 on land and 1 × 2 at sea.

The colour proportions are: green 50%, gold 24%, white 6%, and black 1%.

—  Original wording from Schedule #2 - Guyana Constitution

Notes:

• Looking at the black and white image (in either of the two scans listed above), it is difficult to distinguish black from red, but there are some subtle white reflections that make it possible to see part of the boundary between the two colours.
• The right tip on the black fimbriation clearly sits in the very center of the flag.
• The black fimbriation appears to be 1.5 times the width of the white fimbriation.
• The text is somewhat blurry; there are some claims on the internet that the "black 1%" is actually "black 4%".
• The colour red is not mentioned in the text, but if everything adds up to 100% it would need to be 16% or 19% depending on whether black is 4% or 1%.

Issues:

• If the right apex of the black fimbriation sits at the midpoint of the flag, then gold+white would need to equal 25%; this conflicts with the given numbers given for gold and white in the text.
• If the right apex of the black fimbriation sits at the midpoint of the flag, then red+black would need to equal 25%; this also conflicts with the numbers because a total of 80% is already allocated to green, gold, and white.

Given the inconsistencies in the numbers, the only reasonable choice is to ignore them altogether.

At some point in time, the constitution was converted to electronic media (probably via OCR). Here are copies from 1998 and 2012:

• updated constitution (1998)
• updated constitution (2012)

The last line of Schedule #2 now reads:

The Flag is designed to be of a proportion 3 * 5 on land and 1 * 2 at sea. The colour proportions are green 50” gold 24” white 67” and black 1”

Notice how 'white 6%' got converted to 'white 67"'. 'black 1% is now 'black 1"'. Red is still missing. Not helpful.

The 1998 version of the constitution shows a colour image with white and black fimbriations approximately equal in width.

The 2012 version of the constitution shows a colour image with the black fimbriation about twice as wide as the white one. The corners of the flag are not drawn correctly which makes the entire image seem illegitiment.

There is no evidence that either of these colour images (1998 or 2012) were ever approved by an Act of Parliament.

The flag was designed by Whitney Smith. There is an old photograph of him holding the flag here: https://guyanachronicle.com/2016/11/24/designer-of-golden-arrowhead-passes-on/. Again we can see that the white fimbriation is thinner than the black one. The flag in that photo does not seem to be either 3:5 or 1:2, but that may just be due to camera angles.

Ultimately this construction sheet is based off of measurements from the black and white image from the old printed copy of the constitution.
The flag has been converted from 1:2 to 3:5 by changing the length of the flag while keeping the width of the flag and the width of the fimbriations constant. The black fimbriation is given a width of 1/20 hoist. The white fimbriation is given a width of 1/30 hoist.

For what it is worth, the colour proportions of the flag on this (3:5) construction sheet and the matching (1:2) construction sheet are listed here:

• 
  ratio 3:5 — for use on land
  (this file)
• 
  ratio 1:2 for use at sea