For any method, step one is always, MAKE A TENSION / GAUGE SWATCH.
You will need to know:
How many stitches are there in an inch of your swatch?
This is your STITCHES PER INCH: __________
How many rows are there in an inch of your swatch?
This is your ROWS PER INCH: __________
A pattern that has diagrams with exact measurements is the easiest to work with. You simply multiply the horizontal line measurements by your stitches per inch, and the vertical line measurements by your rows per inch.
(Stitches go left to right, and rows go up and down.)
If you are using a hand knitting pattern, you need a calculator to re-calculate the stitches and rows given. I find the easiest way is to photocopy the pattern, and write the new number in the line space above the old figures. If that seems confusing, white out the old numbers, use the original to fill in the new, as follows.
Divide your stitch gauge by the pattern stitch gauge to get the multiplier. (your gauge divided by pattern gauge = multiplier.)
Example: Your swatch is 9 stitches per inch, the pattern gauge is 8 stitches per inch. 9 divided by 8 = 1.125
Now everywhere the pattern gives stitches you multiply by the multiplier (Example = 1.125)
If the pattern says cast on 100 stitches, you would multiply 100 x 1.125 = 112.5 stitches. You decide whether to use 112 or 113. One stitch usually doesn't matter much.
If the pattern says decrease 7 stitches for the underarm, the formula would be 7 x 1.125 =7.875, decrease 8 sts.
If the pattern says the shoulder is 24 stitches, use 24 x 1.125 = 27 stitches.
Now we do the same thing with the row gauge.
Divide your row gauge by the pattern row gauge to get the multiplier. (your gauge divided by pattern gauge = multiplier.)
Example: Your gauge is 12 rows per inch the pattern gauge is 15 rows per inch. 12 divided by15 = 0.8
Everywhere the pattern tells you to knit rows, you multiply by the multiplier. (Example=0.8)
If the pattern says to knit 10 rows, use 10 x 0.8 = 8 rows.
If the pattern says to knit 33 rows, use 33 x 0.8 = 26.4, decide whether to knit 26 or 27 rows to fit the pattern.
The only time this is tricky, is when there are complicated sleeve increases and decreases. Try to make them match the rows to be knit. Often you may have to double up on the decreases in order to have all the decreases finished by the last row.