Recognising the problem is half the battle
If you want to knit a model but can't quite come to terms with the instructions in the knitting pattern, you can quite easily convert the pattern. First of all, check where the problem is.
The most important thing before starting to knit is to check the tension. If you want to knit a pullover using the original yarn in one of the sizes given, then compare your tension check with the tension given in the instructions. If these two do not correspond, then read the Knitting Tip TENSION SAMPLE.
If you want to change the size, the pattern or the yarn then you have to do some preparatory work to ensure the result is perfect.

The right size
You are well advised to compare the details of the cut diagram from the selected pullover with your own favourite one in your wardrobe. If you would prefer to have it longer, shorter, wider, tighter, then draw a new cut with your own measurements - it's best 1:1. This way, by laying your work on the cut pattern, you can always check that everything is okay while you are knitting, and if necessary change something. With this you also have the perfect template for stretching the pieces before you make up the garment.

Rule of proportion
Now all you have to do is convert the measurements into stitches and rows. That's where the rule of proportion comes in. Your tension sample shows how many stitches and rows you need for 10 cm.
Tension sample: 16 stitches and 24 rows = 10 x 10 cm
Example cut diagram:

You need 16 stitches for 10 cm; 1.6 stitches for 1 cm, and five times as much for 50 cm (wide front and back), so 80 stitches. Don't forget the border stitches. In the length you need 24 rows for 10 cm; 2.4 stitches for 1 cm, and four times as much for 40 cm (length from the rib to the armhole for the front and back), therefore 96 rows.
The conversion formula is: (cm x tension sample) divided by 10

Now you can determine the benchmark data for your garment by converting all the measurements width-wise into stitches and length-wise into rows.
If your calculations don't result in a whole figure, then round up or down, as you see fit. In the final result, you normally can't tell the difference.

Shaping sleeves, armholes and necklines
For increasing to shape the sleeve you have in our example 7 cm (11 stitches) in the width and 41 cm (98 rows) in the length. Divide the number of rows by the number of stitches, and as a result you will have to increase one stitch at both ends of every eighth row. In the last two rows before the armhole or before casting off (where there's no armhole shaping) nothing happens.
For shaping the armhole and the neckline, lay your work directly on the cut pattern and decrease accordingly. It is also helpful to go by the original instructions - the differences are normally not so big.

Repeat patterns, knitted motifs and division of space
If you are knitting repeat patterns (ribs, cables etc.) you should take the number of stitches and rows into consideration. Sometimes the only solution is a compromise between measurements and patterns. Either the measurement or the pattern has to be adapted. Stick to the original pattern instructions. A comparison of the various sizes can often help here.

Knitted motifs or division of space should be placed on your own cut pattern and then calculate the exact position with the rule of proportion, or place your work on the cut and go by the drawing.

Another tip
How important the tension sample simply can't be stressed enough - and always before you start! Every next deliberation and, when it comes down to it, the entire success of the whole thing depends on it. But knitting is supposed to be fun, so don't turn it into a Ph.D.! A little improvisation is always allowed....