Use your common sense in assembling your model. The instructions are usually written in a certain order for a reason. An exception may be if you add additional detailing. You might have to detail extras before assembly. Assembling small parts may require the use of tweezers or toothpicks.
Toothpicks also work well for applying small amounts of glue.
Filling seams and cracks on a model will enhance its apperance if done properly. Apply putty along the seam and build up a little ridge. Use your finger dipped in rubbing alcohol to smooth wet putty. After the putty dries, sand until the putty blends with the rest of the model. Remember to prime the putty before final painting.