If you are a planner:
Try to have an idea in your mind or on paper of what you would like to see. For instance, you might like a center structure or on the sides or caves & tunnels. Then look at what pieces you have & start by using some bigger pieces as your base (for stability). If you haven't filled up your tank with water yet, it's a lot easier (& drier) to do it with the tank empty. Just keep moving LR around until you get the look you want. Try to work the rock together so it's stable but has the holes & crevices for your creatures. If you planning to have corals on top, then you should also plan for this, but most of the time you'll be able to find plenty of flat areas & holes to stick frags & corals into. Also, consider how much sand bed or bottom you want or need. For instance, if you're planning to keep sand sifters (fish or starfish) then more open space is preferable. Planning ahead also helps you when you're buying new LR so you know whether you want flat pieces or Tonga branching or what.
If you're not a planner:
Just wing it & have fun.
This is not rocket science. It's purely about what you think looks good.
Anthony