A scheme is one entry per datastore holding the datastore, the field that receives the number, the prefix, the padding width and the next value to issue.
Choosing a prefix
Keep them short, upper case and distinct. Their whole value is that somebody reading INC-000042 immediately knows what sort of record it is, so no two datastores should share one.
Choosing a width
Six digits covers just under a million records before the number grows a digit — at which point references stop lining up neatly in reports, but nothing breaks. Pick a width that comfortably covers the datastore's lifetime.
Starting from something other than 1
Set the next value when you create the scheme. This is what you do when migrating from a previous system: start above the highest number already in use, so old and new references cannot collide.
Changing a scheme later
Changing the prefix or width affects only future records. Existing records keep the numbers they were given, leaving the datastore with two formats in it. That is often acceptable — just decide it deliberately.
Existing records are not numbered
Creating a scheme does nothing to records that already exist. If you need them numbered, do it as a one-off exercise and then set the next value above the highest you assigned.