-The basic idea when you see a Tetris symbol is that you must create an area bordered by the line or the walls of the grid that represent the shape. The symbol itself must be within this shape, it doesn't matter where.
-The shape you draw with the line must be exactly as shown by the symbol. You cannot change the shape in anyway except for two situations which I will cover next. But what this means is that a three block vertical line must be drawn as a three block vertical line, not horizontal. Same goes for any of the non-standard shapes which have gaps: the gaps must be represented exactly as shown in the symbol.
-One of the corollaries to the above rule is when the symbol is slanted. When you see this, it means you can rotate the symbol. Keep in mind that when rotating an L or a J and other such non-symmetrical symbols, they cannot be mirrored. An L may be rotated to horizontal with a hook on the upper-right or lower-left, but not the other two positions.
-The other exception are hollow squares. These are a separate but related symbol that subtract the need to include exactly one square in the shape they are enclosed in. Hollow squares often come grouped together, but the shape they are shown in does not matter. You merely need to subtract as many squares from the shape you draw as there are hollow squares enclosed within it. It is possible for hollow squares to subtract all squares from a Tetris shape, in which case it does not matter what area they are confined in. Note that you can't have more hollow squares contained within an area than there are Tetris squares to subtract: you can't go into the negative!
-When there are multiple symbols to be used, it is possible to combine them into one single continuous area. To understand this it's best to think visually, and pretend you're playing Tetris. Essentially, pieces can nest together to form larger shapes. When pieces are grouped like this you do not need to draw a line between the shapes. For instance, suppose you have this J and T symbol (represented by o's, x's are blank spaces):
xo
xo
oo
ox
oo
ox
The two shapes can be nested together as such:
oxo
ooo
ooo
You would only need to draw a line around the outside of this combined shape! This also works in situations where shapes merely touch instead of necessarily nesting. This L and J:
ooo
xxo
xxo
ooo
Can combine to the single shape:
ooo
xxo
xxo
ooo
Combined shapes can get much more complex than this of course and may involve as many symbols as the grid can fit.
-Now, the big thing many people seem to miss at first: in such combined groups, it does not matter where the individual symbols are. The symbols for the J and T or L and J above could be ANYWHERE within their combined group. They don't have to be overlapping their actual shape in the group! So long as the symbol is within the lines, it's good.
-Lastly, a quick sanity check method for when you're having issues and don't know why. Count the number of squares in the Tetris shapes you are working with and then count the squares enclosed in that group. If they don't match, you either have too many or too few squares enclosed.
