Depending on your venue and the type of music you play, (I take it you would be middle of the road/country/jazz/rock?) I would do the following.
Set the PA speakers up according to available space on stage or the rehearsal area... keep them in front of the vocal mics and as far apart as possible either side of the stage.
Drums:
1) Get the drummer to tune his kit, place one of his mics over the hi-hat /snare area, one between the first flying Tom and floor drum with consideration to using "side pickup" of the crash and ride cymbals..and finally muffle his kick drum and place the mic either inside or mounted to the front of the kick drum.
Have the drummer work around his kit while adjusting the mic settings at his 3 channnel onstage mixer for each area he has to obtain a even level thru the main PA...
once set, the drums can then be linearly raised and lowered at the main PA desk.
You are listening for a clean unmuffled full sound when raising the main drum volume at the PA mixing desk with no feedback at the highest output level he is likely to play at depending on the drum sticks/brushes selected for each song.
Set the guitar amps in the usual order onstage, if you have full foldback facillities, I would use the line out from the guitar amps and have all the guitar amps onstage set as low as possible (guitarists love to hear themselves ..ay Meka
)
You can use the guitar amps to provide "onstage foldback" to each muso if you don't wish to use the foldback, but be aware the operator doesn't raise their own levels independantly while playing if you use mics as pickups for each guitar amp.
Run the guitar line outs to the PA desk and have each guitarist set their prefered onstage amp level (as low as possible) and then raise the PA mix level to test for the required sound thru the PA.... repeat for each guitarist .
Set the Vocal mics up for each singer... ensure the Mics are behind the line of the front of the PA speakers... use the graphics equalizer and echo to obtain the best sound for each singer's preference and the clarity as heard at the desk thru the front PA speakers.
Place the foldback speakers in front of each singer and the drummer as required... I doubt you will need to direct a high level of instrument sounds thru the foldback if you are using the guitar amps to provide onstage sound
Have the singer(s) test the mics again....raise the main PA level to the max volume level for the venue while he "sings" into it....listen for feedback and adjust the volume/graphics equalizer settings to suit.
Set all levels on the PA desk to low and have the band play a song that would be the loudest in your repertoire...
Have the mixer raise the main PA levels and mix the sound to obtain the best performance.... adjust the foldback levels as indicated by the misicians while they play....listen for feedback and use the foldback graphics equalizer to eliminate it.
Simplified...that's usually does it for us.
