In the "Little Nemo" skit, two people use their arms and one person's head to create the character "Little Nemo." The character is meant to be funny, as he is acting as an adult, just in a small body. In putting your skit together, rehearse the script several times to make sure that the character is funny. Also, make sure that the sheets and table stay in place through your various scenes.
Find a location to set up Little Nemo, such as a doorway. Two people will need to be placed in close proximity to each other, so a doorway provides sufficient space.
Configure two or three sheets so enough space is available to have one person's head and arms, along with another person's arms through the whole between the sheets.
Place a table behind the hidden sheets. Have the first person lay on top of the hidden table, placing his head and arms through the hole in the sheets. This person users his arms as Little Nemo's torso and feet. Place the first person's hands into the character's jeans and shoes.
Place a second person below the table. This person will reach around the person on top of the table and use his arms as Little Nemo's arms. Have this person insert his arms through the shift, reaching around the person above the table.
Practice with a script before debuting the character. For example, pratice a funny dance move or comedy routine that you come up with. You can also have the character do various exercises, such as jumping jacks.