Jon - the decor panels are usually wider than the cupboards, this is so you can 'scribe' them into an uneven wall.
Method - hang the cupboards on the walls, check with spirit level & adjust as necessary. Next, offer-up the decor panel to the cupboard end & wall (keep the front edge of the panel vertical) then clamp the panel to the cupboard (scrap wood under the clamps so as not to mark the panels). Go to your old school geometry set, select a compass (complete with pencil), set the distance to equal the gap between the decor panel and the wall. Carefully slide the compass point up the wall so that the pencil marks a line on the panel - this line will be parallel to the wall. Un-clamp the panel & then cut along the line; this panel should then be a perfect fit to the wall & front edge of cupboards.
Take care when cutting the panels so to avoid damaging the laminate surface.