"I'll tell you how it was" is the correct one.
This sentence is in the future simple tense: "will tell".
Short answer: "I'll tell you how it was".
Longer answer: Imagine a timeline. "When I come back from my trip" takes place further along the timeline. Now is your trip before, after, or at the same time as that point? Since the trip is something that (at the future point in time) you will have already finished, it in the past tense.
The other three (it is/will/should) all deal with time points at the same time or after; but if you have come back from the trip, normally it should already been done ! :)
In all, when in doubt, draw a timeline, and usually that should make things clearer :)