With impression dies, you can choose to press either solid or hollow (with sheet metal).  Some dies lend themselves better to one way or another, but for the most part, you can press either solid or hollow with any impression die.  Pressing solid means you have enough metal to fill the entirety of the impression/die.  Pressing hollow means you use a conforming part (urethane, paper, cardboard, leather, etc.) to force thinner sheet metal into the die.  The sheet metal is shaped down into the impression and the forming can be seen on both front and back.  Typically, we suggest pressing shot plate dies and ring dies solid.  Dies with a deeper impression usually work better with sheet metal.  The image below shows the difference between pressing a die solid or hollow.

When pressing solid, you want to melt an ingot of silver or use sheet that is thicker than the deepest part of the impression die.  Use a tool steel pusher on the upper platen to force your metal into the die.  Do not use urethane. Once you have pressed you can cut the excess metal away from the impression.  With many dies, you will get all the detail from the die in a single pressing, however, some dies will require that you anneal, trim away excess metal, and then press again (and maybe repeat that process 2, or 3, or 4 times until you get the level of detail you desire.

When pressing hollow, with sheet metal, you will start with an annealed piece of sheet that is slightly larger than the area of the impression.  Deeper dies will need more excess metal than shallow dies, to allow for the metal to be pulled into the die.  Use small pieces of urethane and a tool steel pusher to force your metal into the die.  It is helpful to press, then move the urethane to another area of the impression and press again (repeating this process a few times).  It is also helpful if the piece of urethane is smaller than the impression.  Remember, urethane flows under pressure so it will squish out and press the metal into the die.

Try pressing both solid and hollow to see what works best for you!  And remember, each die is different so what works well with a particular die won't necessarily be the best course of action for another die.  Experiment and take notes on what works best for you.