1) Create the wad, by using SRB2 Doom Builder, like rocky said.
2) I hope you know how to handle wads editors like XWE or Slumped ; open a WAD editor like those, copy all the lumps from the wad you created with SRB2 Doom Builder, and past them into this same wad. Then, you'll have two maps. Select again all the lumps in your wad, and past them again in this same wad, and you'll have 4 maps...then 8 maps, etc.
Don't forget to rename the MAPXX lumps according to the level you want to replace (MAP02 for GFZ2, MAP05 for THZ2...)
3) You'll be able to edit the levels when you'll open your wad with SRB2 DB : it will ask you which map would you like to edit : MAP01, MAP02...
... if you had any troubles with the vocabulary of the wad building, then it means it's too soon to start a pack, trust me please -