First, add a couple bowls to your board. These can hold your spreads, some of the smaller fruit like raspberries, mini chocolate chips, or any other elements that might be smaller and likely to spill or need to be contained in some way. You want to build around the bulkier elements for a visually enticing s'mores dessert board and the circles of small bowls help break up the typically square shapes of chocolate and graham crackers.
The next thing to do is to create some sort of visual movement that draws your eye around the board. I typically like to do this with curving rows of graham cracker cookies that have been broken in half. Try for "S" or "C" shapes that curve around the bowls already placed on the board to help pull eyes around the board.
Now it's time to start adding any larger elements like jumbo marshmallows, chocolate bars, and fruit. Try to think about color contrast to keep things pretty and the board looking balanced. For example, if I am using both strawberries and raspberries, I will place them in separate areas of the board with chocolate or marshmallows in between to let their colors pop visually rather than sticking them both together.
Lastly, fill in any gaps with smaller items. Mini marshmallows, mini peanut butter cups, and Andes mints are all good options for closing up any empty spaces to the board looks full and generous.