Seam allowance is what you add around the edge of a pattern so that it can be sewn together. If you do not plan to make up a toile of your pattern (in the case of blocks), then there is no need to add seam allowance (when you are using a block to create a pattern it is much easier to use it without seam allowance and then add seam allowance once the pattern is complete).
I find the easiest and quickest way to add seam allowance is with a clear grading ruler (don’t worry If you don’t have one, you can just use and ordinary ruler and mark the seam allowance width at intervals along the seam and then draw the line through all the points).
Take the pattern that you are adding seam allowance to.
Decide on how much seam allowance you will be adding (if you need help with how much to add look at the table at the bottom of this post) and find where the line is that indicates that width on your ruler.
Lay that particular line (the width of your seam allowance) along the side seam of your pattern.
With a sharp pencil, or pacer, draw in your seam allowance, being careful to keep your ruler in place.
Be sure to extend the line past the original line by a couple of centimetres (this helps when we add seam allowance to the other seams).
If the seam you are adding seam allowance to is curved you will need to mark the seam allowance with a broken line. Line up your seam with the ruler and draw a small line.
Then pivot you ruler to you next point (I tend to do this every 1 – 1.5cm) until you have gone around the whole curve.
Draw in your curve by joining the broken lines to form a smooth curve. You can do this by carefully pivoting your ruler, using a French curve or something else round (like a large mug or plate depending on the shape of the curve). Once again, remember to extend this line a couple of centimetres beyond (in this case beyond the shoulder line) – as it helps when adding seam allowance to the next seam.
For tight curves (such as the bodice neckline) mark your seam allowance guidelines closer together to ensure a smooth and accurate curve.
Work your way around your pattern piece, adding seam allowance to all seams (apart from sides that are “PLACE ON FOLD”).
How much seam allowance to add is somewhat up to individual preference. Depending on the garment you are making and the fabric you are using, the seam allowance you require may change. For example, if you are making a silk chiffon top (which is a sheer fabric) it is best to use a narrow seam allowance, so you are not left with bulky seams that show through. A silk chiffon top is a delicate piece of clothing, that is not worn everyday and is normally hand washed, so it can afford to have smaller seam allowances. But, if you are making a pair of trousers or a coat, you need seams that a stronger (particularly in places where tension is put on the seams - e.g. the crutch of trousers) and therefore wider than what you would use for your chiffon top.
I have put together a table to help guide you with how much seam allowance to add, but as I said, it is up to you!