It’s definitely a two-person job — two people besides the baby, that is. One adult should hold the baby securely and comfortably. The second person must firmly press the handprint into the printing medium. Press the hand in all at once rather than gradually so there’s less chance of sliding or smearing. If you prefer, you can try to make a print while baby is sleeping. This method may be easier to execute (as long as the child doesn’t wake up) but will require awkward angles that might not create the clearest impression.