Among all the mandala symbols, the Sri Yantra (known by some as the Sri Chakra or Chakra Raja) is considered the most fundamental or "prime" mandala. Yantra is a Sanskrit word that can be translated as "Power Diagram" or "Instrument," Sri Yantra as "Holy Instrument" or "King of Power Diagrams" depending on who you ask. (When referred to as the Chakra Raja, the term translates to "King of all Chakra"). It's both symbolic of Hindu Tantra and a core part of the Shri Vidya subset of Hinduism, adherents to which make the Sri Yantra an object of worship.
The central figure of a Sri Yantra consists of nine triangles, interlocking and connected to each other at their common points. The four isosceles triangles pointing upwards represent the masculine form or Shiva. The five isosceles triangles pointing downwards stand for the female form or Shakti. (The Sri Yantra therefore can also be seen as standing for the union of male and female divinity.) The center "power point" (bindu) stands for the invisible center that is the origin of both the figure and the cosmos, while the interlacing of the nine triangles forms a web of 43 more triangles: a web meant to symbolize the complexity and interlocking nature of the cosmos, expressing non-duality (Advaita). Surrounding the center are two lotuses of eight and sixteen petals (the lotus of creation and of reproductive vital force, respectively) as well as the "earth square" reminiscent of a temple with four doors.
While it seems simple to draw when you first look at it, the common connections between the triangles make drawing the Sri Yantra exceedingly difficult without running into errors at the intersections. Sometimes it's concurrency, where the triple intersections fail to come together at one point. Sometimes it's a problem of concentricity or centeredness, where the "bindu" isn't at the center of both the innermost triangle and the outer circle. And sometimes the problem is as simple as the innermost triangle not being equilateral. Making matters even worse for the student of sacred geometry, there are many different variations in the way the Sri Yantra has been drawn over the years. This is probably because the best-known historical methods for drawing this intractably difficult figure were quite imprecise and a little incomplete.
However, modern researchers have largely solved the problem of creating accurate Sri Yantras. We now know three different forms: the flat, the pyramidal, and the spherical. Both the flat and the pyramidal date back to antiquity... the pyramidal just comes from interpreting the Sri Yantra as having nine levels (at which point it's called the Nav Chakra). The spherical is by far the hardest to draw, and even today is mostly drawn with computer aid. However, drawing a flat Sri Yantra is easily done with pen, ruler, and paper if you have access to modern instructions. After 12,000 years of research, it's finally easy for anyone to re-create one of the most famous and certainly one of the oldest mandala symbols.