I know that Rusty did one for his church and gave some sage advice. hopefully he'll respond.
I just finished one 24x36 inches, 300dpi with pics of Patti's face painting. We're going to Staples today to get it printed. My approach was:
Make a main layout of 24x36 with the background as I wanted it. Save itas psd.
Do the rest in sections ( quadrants, 1/6ths, etc.) as separate pics. That saved in processing (particularly saving) time. I saved each section as a psd with all pic's as individual layers and transparent background. Then I did shift-cntl-alt-E on the top and got a merged version as the top layer. I then did a copy-paste of that combined layer and pasted into the main layout.
Once I got them all on the main layout and saved as psd, each section on its own layer. I then had to do fine tuning. It involved having to go back and change some of the sections and re-pasting on the main layout. You'll know what you have to do pretty quickly.
Once I got them to my and (for sure) Patti's satisfaction, did the shift-cntl-alt-E and got a merged version on the top. I saved as psd. processing time took a while because of the size of the file.
I then copied the top layer (the merged one) and pasted it into a new document and saved that as a jpeg.
I copied onto a USB flash drive and am hoping for a successful printing. I'll let you know.
Obviously this is a somewhat sanitized version of what really happened, and reflects a learning curve.
Things to keep in mind:
Make sure you know just what size you need to be seen. If there is to be frame, take that into account in your layout. The actual visible area of the pic's on ours will be 22x34 because of the frame.
Make frequent checks of portions of the pic at actual size. I needed to do that to make sure all was consistent.GOOD LUCK