The edge that goes into the printer first must be a staight, square edge. So, with my first decals on a sheet, I print them along the top. I then cut the top strip off. For my second print, I merely turn the sheet 180 degrees, and then I am putting one of the original sheet edges in first, so it will feed fine (decals still laid out along top of sheet).
If I get three sets of decals from the sheet, I then have to square up one of the edges to ensure proper feed.
Sometimes there is enough of the sheet left to do the amount of decals that will fit on the face of an envelope. I then cut the sheet to the size and shape of a business envelope, and set the sheet size in the printer as envelope. Some printers print envelopes widthwise, others endwise, check your printer manual. Makes no difference to the decals.
Eventually, you will get a remaining piece that is just too small for the printer to handle it- I keep a few of those around to do little pieces where I need to make a solid-color decal, like for hatches and stuff. These I then trim with hobby knife after printing and sealing.