Deck framing isn't as easy as it may appear. If you're a few inches off and don't notice, it shows up when you place the deck boards. Not to worry. It happens all the time. You can place the deck, check the fit and make some minor adjustments. Nobody will know you tweaked parts to fit.
First Deck Board
If your deck frame is finished and ready for deck boards, start by placing the first board. Chances are that most of the boards will lay square until you get to the outside edge on the opposite side, where the out-of-square framing tapers off at an angle. The angle will likely not show up until you've placed most of the deck boards. That's fine, you can deal with that later. Cut the first deck board to length and place it parallel with the home or building on the deck framing using the desired spacing between the wall and the deck board. Spacers work great for this purpose. Place a few wedges between the wall and deck board and slide the deck board up tight against the wedges.
Check the Ends
Cut a miter or angle on the end on the deck boards if needed. If the ends of the deck are not square, the deck boards may not fit right, or they won't look right. Make a few test cuts, angling the saw slightly to cut a slight angle on the end. Place the board and check the fit. If it's acceptable, lock the blade on the miter saw so you can cut adjoining deck boards at the same angle. Depending on how much the deck is out-of-square, both ends of the deck boards may need to be cut with different miter angles. The boards may have one square end and one angled end. Cut some short deck boards with different angles and use them as test pieces. When the angles are right, mark the blocks for reference.
Place adjoining boards side by side across the deck beside the original that you placed parallel with the building. Place one or two screws in each board, just enough to hold them in place. When you get to the last board and it won't fit because the angle is wrong, drive small wedges in appropriate places to widen some of the gaps. The cumulative effect of the wider gaps may be enough so that the outside board fits flush along the edge as though the deck were square. If the wedge method is not enough, place the board on the deck, trace an angled line on it as far as needed to fit the space. Use a bandsaw or circular saw to trim the edge off slightly so that it fits. It's unlikely that the angle will be severe or even enough to notice. If the deck is that far out, consider adjusting the framing or rebuilding the frame.
There might be one other option. Instead of running the boards parallel with the length of the deck, in some special situations you may be able to cut them shorter and run them perpendicular to the length of the deck, provided the boards are adequately supported by the joists and/or structural blocking. This likely means you'll need more blocking and bracing, but it might be worth the effort to compensate for a badly out of square deck. This can actually save you money, because you're using shorter material, and if the deck is out-of-square lengthwise, all you need to do is miter the ends of the short boards or cut them off at slightly different lengths and they will fit.