First time DM needs math help
I'm new to D&D within the last two-ish years and loving it. I'm part of a longer campaign where our current DM has really enjoyed my character building and background. They're looking to take a small break from our campaign this summer and have asked if I wanted to DM a mini campaign in the meantime.
I have a rough story and setting idea that I think will be a lot of fun. It involves a Cabin in the Woods type "twist" where all of the PCs turn into creatures from the Monster Manual that they will unknowingly choose. I'm struggling with the math behind it though.
I know CRs vaguely mean a party of 4 of that level could reasonably defeat the creature. But we'll have a party of 5, all with Monster Manual CRs and I'm not sure how the math would work there...
Anyone have any tips of how I could weigh some of these numbers to make for a fun, challenging, but doable encounter? Thanks in advance :)