So, there are two main methods for making clothes - draping and flat pattern-making [link]. With draping, you work primarily in 3D. With flat pattern-making, you work primarily in 2D.
The flat pattern-making method is made easier with the use of templates (a.k.a. slopers or blocks) [link]. Blocks are made from measurements of the human body. They need to fit perfectly because all other patterns will be based on them. Blocks are difficult to make, but once made, the rest is a lot easier.
So how are blocks made?
Although blocks are used in the flat pattern-making process, to make the blocks themselves you can use either draping (make them in 3D) or flat pattern-making (make them in 2D). IMO, making the blocks using draping (i.e., in 3D) yields better results. You see exactly what you´re doing, you understand what is going on, idiosyncracies in your avatar´s physique are taken into account because they´re right in front of you, etc. Making the blocks using flat pattern-making (i.e., in 2D) has always given me blocks that don´t really fit well and need further adjustments.
Information on how to make blocks (and, to a lesser extent, patterns) is usually not available on the web, at least not for free. There are a couple of places you can look at, though.
MAKING BLOCKS USING DRAPING
[Link] This is the only site on the web as far as I know that offers detailed instruction on how to make all basic blocks using the draping method. The lessons Preparing The Form, Basic [Two-Dart] Bodice, Drafting The Sleeves, Basic Straight Skirt, and Basic Pants contain all the info required for you to make all major blocks for your avatar.
If that link goes dead, this book [link] is a great resource for making blocks using the draping method.
Draping is a long process. Moreover, draping in MD is currently difficult (pins are buggy and cumbersome to use, there are no scissors, fabrics stretch a lot). However, the results will be very good.
MAKING BLOCKS USING FLAT PATTERN-MAKING
As I said above, making blocks using this method has always given me blocks that don´t really fit well. There are a couple of places on the web that offer instruction on how to make all basic blocks using the flat pattern-making method. One place is this [link, in particular 1,2,3,4]. Another is this [link] - click to download the measurement generators, they are Excel files.
Some pattern-making books (not all) also offer instruction on making blocks using the flat pattern-making method.
MAKING BLOCKS AUTOMATICALLY
Hat tip to marcuswilm for telling me about this [link]. The Cutter is a block automation program. The demo version includes all the functionality a MD user might need. The program makes blocks using the flat pattern-making method. As I said before, I´ve never gotten good-fitting blocks using this method...and that held true with this program too.