Patterns that are within a gament are often called "motifs", and we will call them that here.
If you're making a flat-knit piece, it is a little easier than if you're making a project in the round, or even a garment with sleeves. Let's start there.
Above is the Flower Chart from the Soria Vest by Mari Muinonen, originally published in Knitty, Winter 2009. This is a beautiful, long pattern repeat (motif). If the gauge is 5 stitches per inch, this covers over 3.5 inches.
First, take a long look at the motif you want to use. Looking at the chart above, you can see that the column of stitched marked "1" is a connecting column. It isn't part of just one flower; it's how you would connect several together. If you only want one of the flowers, you need to ignore this column entirely, making your pattern now 17 stitches wide. (I tend to hide the unused column with sticky notes so I won't get confused.) Because 17 is an odd number, centering it pretty easy. You would center it on the column numbered "10" of the motif chart. How do you center it on the garment? Let's say the garment is 51 stitches wide.
Centering = total number of stitches - stitches in repeat = # of background stitches.
# of background stitches/2 = # of stitches to knit before inserting your repeat, and the number of background stitches to knit after your repeat.
or 51-17 =34. 34/2 = 17, so there would be 17 background stitches followed by the flower pattern, followed by 17 more background stitches per row.
That wasn't so hard. But what if you wanted three flowers across? This is when it helps to photcopy, cut and tape charts together, or grab some knitting graph paper and draw exactly what you want to do.
In order to get the pattern centered, we've already determined that we need the flower (without connectors) to have 17 stitches on either side of it. The flower will need a connector on each side to join it to the next flower, making 19 stitches wide. This leaves 16 stitches before and 16 stitches after the centered flower. To make this work, you need to leave off the first two columns of the pattern in the first flower, then follow the whole chart once (connector plus flower), then follow the first 17 stitches of the pattern again for the third flower (connector plus 16 stitches of the flower).
When you draw this, start with a graph 51 stitches wide, and start on the center stitch (number 26). You're going to draw this so that if you fold the paper in half lengthwise, the stitches to the right of stitch 26 will mirror the stitches to the left of it. Center just the first flower. Next, add a connector column to each side. Then, add the left 16 stitches to the right side, and the right 16 stitches to the left side. See how nicely that worked? Now you have a chart representing exactly the stitches you plan to knit - there's nothing left to keep in your head. Just follow your hand-drawn chart.
The centering equation will always work, unless you end up with an odd number of background stitches. When this happens, you can usually add one stitch to the total number of background stitches so that it will balance from side to side. If you can't even out the number of background stitches, you'll have to decide whether you can live with it being off by one stitch, or whether you need to choose a new motif.
Next time, I'll tell you about combining motifs, as you would in a Fair Isle type garment.