it's all iffy with 1 basic thought...and others variable...
ie., 1 blows in, 1 blows out...the others you can adjust but achieve a balance
peter says to blow in from the bottom...whereas I would say blow out...only because dust settles to the bottom and I would not want to suck dust up from around the case
....BUT the vent on the bottom may just be a vent and not a place for a fan...so keep that in mind
top slot is good for sucking air out
the back one should suck out...
so here's a quick breakdown of how I would do it if i was to add more fans..including the ones you have already...
front suck in
back suck out
nothing on bottom
top suck out
side 1 suck in -- if vent is near or over cpu
side 2 suck out --
I use "suck" because I think it's more clear in an illustrative sense