The following describes the flat pattern-making process. See this for more info.
In flat pattern-making, making patterns is not hard because patterns are based on templates. The hard part is in making those templates (known as slopers in the US and as blocks elsewhere). They look simple (because they are templates afterall), but they are hard to make. They require many measurements of the human body and a lot of test-fitting. They need to be perfect because all other patterns would be based on them. Once those blocks are made, though, the rest is significantly easier. You don´t need to measure the figure anymore, or even test-fit new patterns on it. You just use some geometric rules to derive different patterns from your blocks. And you can be assured that, just like your blocks, they would fit perfectly too.
The following are examples of patterns made from blocks. (Scanned from the Italian pattern book Il Modellismo Sartoriale.)
These are the skirt blocks that the author of the book uses to derive all other skirt patterns. The front skirt is on the right and the back skirt is on the left. He uses one dart for each piece (instead of two) and also includes a little flap at the back for ease of walking (the little rectangle at the bottom of the back skirt).
Now, this is a wraparound skirt. It is derived from the previous blocks with just a few simple operations. Unfold the front skirt, delete a section of it, and remake its edge (the red line). Then, give the skirt a little flare (at points C1 and C2). Lastly, delete the back flap (not needed anymore since this skirt is easier to walk with). As you can see, if you already have the skirt blocks made, making this style of skirt simply takes a few minutes. And it will fit just as well as your blocks did.
This is a flared skirt without darts. It is also derived from the skirt blocks posted above. For this style, slash each block along the dotted line up until the dart, splitting the block into two separate pieces. Then, pivot the pieces around D1 closing the dart and opening--flaring--the bottom of the block. Also, as with the previous style, give the sides a little flare too.
This is a miniskirt with a Western-style yoke-belt that´s continuous from front to back. It is also derived from the skirt blocks. I will not attempt to explain how it´s done in words, but it should be fairly obvious. The final patterns are three...the yoke-belt, the bottom of the front skirt, and the bottom of the back skirt.
Things can get a lot wilder too. This is a skirt with a novelty waistband and front pleats. It is also derived from the skirt blocks.
And, this is a wraparound skirt like the one we saw above, but with a draping effect added to it. Also derived from the skirt blocks.