As you in the first set of instructions, you can determine the letter that goes in a specific cell through process of elimination. You will rarely find a cell with all 8 alternatives ruled out, but knowing even only most of the eliminated options will give you lots of useful information.
Consider the following partial sudoku grid:
There are still three unfilled cells in the right hand box (A8, C7, and C8). However, the 7 in A4 means that A8 can't be a 7:
This means that either C7 or C8 must be the 7. Even though you don't know which of the two it is, you should still mark the pair of possibilities:
… because this shows you where Row C's 7 will be. This then means that C1, C2, and C3 cannot be 7. In addition, the 7 in cell A4 tells you that A2 can't be the 7 either. And A1 and A3 are already taken:
This leaves only one possibility for where the 7 goes in the right-hand box:
Now the 9 in C9 shows you that C1, C2, and C3 can't be 9:
. . . leaving only one possibility for where the 9 will go in the left-hand box:
Notice now that only 3 cells in the right-hand box remain empty. We don't yet know which number will go where, but we know what those three numbers will be:
We can do the same thing with the 3 empty cells in Row B:
Again, we don't need to know which numbers go where in the C1-C2-C3 trio or the B4-B5-B6 trio for them to be useful to us. Knowing those options eliminates all but one possibility for cell A8:
. . . which in turn tells you which number, other than 7, must appear in cells C7 and C8:
. . . which lets you know the trio of numbers for cells C4, C5, and C6:
. . . leaving only 2 options for cells A4 and A5:
Notice that we accomplished all this without knowing anything about the columns in other 6 boxes in the full grid (Rows D through I). But we can now use that information to help us figure out which numbers go where in the B4-B5-B6 and C1-C2-C3 trios and the A4-A5 and C7-C8 doubles.
Now you can: