Adding negative ease to a garment is the process of making it slightly smaller than the wearer's body measurements. This is done to create a snug fit that is comfortable and doesn't restrict movement. Negative ease can be added by altering the pattern, or by choosing a smaller size when sewing.
There are two main ways to add negative ease to a garment: by altering the pattern, or by choosing a smaller size when sewing. If you're altering the pattern, you'll need to make sure that all of the pieces are reduced in size equally. The easiest way to do this is to trace the pattern onto tracing paper, then cut out each piece along the new lines. You can also use a ruler or tape measure to mark out the new dimensions on the existing pattern pieces.
If you're adding negative ease by choosing a smaller size when sewing, you'll need to take care when cutting out your fabric pieces. Make sure that you cut them all according to the measurements for the smaller size, rather than using your own measurements. This will ensure that your finished garment has negative ease built into it.