First of all you need Mapedit, depending on the version you have you
v1.11 or Plasma Pak -
Blood Tools v1.21
Put Mapedit in the Blood folder and drag MAPEDIT.EXE
onto the DOSBox shortcut.
If you're an absolute beginner then these tutorials
are a good start.
or watch some videos on the Nyyss0nen
YouTube channel for example.