There are three steps to perform a T-Spin Double:
1. Create the base
2. Create the overhang
3. Execute the T-Spin Double
The creation of the base is often overlooked in T-Spin Double guides.
I recommend reading
this for step 2 and an overview of T-Spin Doubles.
This guide is my approach to creating the base.
Creating the baseTo create the base of a T-Spin Double setup you must create this pattern: Block-Space-Block
There are many variations that can be used to create this pattern from the beginning of the game. The blocks colored gray indicate where the pattern has been formed. Note that many of these formations can be mirrored. J mirrors L; S mirrors Z; I,O, and T mirror themselves.
For some of these it is important to first accommodate the S and Z pieces.
Here are some examples from the previous fumen:
This pattern is very frequently created while playing, even if unintentional.
Completing the base (without an overhang)
After completing the base you will be left with an upside down T shape. You will often have some leftover blocks after creating the base (frame 2).
To complete the base after creating the pattern from the previous section, simply fill in the spaces to the left and to the right of the pattern until the upside down T shape would clear two lines.
It is usually best to complete the base while also creating an overhang. Read
this for help creating overhangs.