Decide on what you want to make. A building has slightly different requirements than a dinosaur skeleton.
Use a photograph to analyze what you want to make from every side and angle if possible.
Draw a scale image of every visible (exterior) part on graphing paper---for a house, draw the outer walls, roof and chimney; for a dinosaur, draw each tiny bone. Make sure your measurements are accurate and consistent so that each piece will fit together.
Cut out the shapes of every section you have drawn. Don't forget to cut out something to mount them on if necessary. For example, your dinosaur will need an appropriately sized spine to hold each rib and vertebra.
Lay out your pieces all together and in order so that you can see what goes where. Build from the inside out---for a house, start with the inside of the house, move to the roof, and then finish with the exterior walls; for a dinosaur, start with the internal bone structure and build onto it while moving out so that you end with the extremities and the head.
Use a quick drying glue that will still allow enough time for correcting any possible mistakes. Modeling glue is ideal.