What are the steps required to revert from STI shifters to friction shifters? I'm curious about trying friction shifters out for my commutes – especially since I had a shifter cable mishap recently that cost me a lot more time than it should have.

My commuter is a '70s road bike on which the previous owner upgraded the stock downtube shifters to STI shifters. In other words, the frame accepts downtube shifters already, but they aren't there. As far as I understand it, the basic procedure would for a permanent conversion would then be to


Install a pair of friction shifters on the frame again.
Remove the bar tape to be able to
Replace the STI shifters with regular brake levers.
Install (??) the shifter cables in the friction shifters.
Cut the shifter cables shorter.
Reapply bar tape.


If I just want to try this out, could I skip step 2, 3, 5 and 6? And just have the friction shifters pull the cable when it's still attached to the STI shifters? (With full knowledge that this will make the integrated shifters ineffective.)

Would this be a lot of work to just try it out? (Assuming I am able to get a hold of a pair of friction shifters.)
Basically you may drop steps 2, 3 and 6.  You'll just remove the shifter cables from the STI levers. It might not please the aesthetes but it will work. If the STI levers have the shifter cables run through external housings you remove these as well. If the housings run under the tape, you leave them alone, you just cut them where they come out from under the tape. (At this point it is a good idea to buy a cable and housing cutter which is a very useful tool that avoids a lot of messy and badly cut brake/shifter cables and housings.)

The downside for keeping the STI levers is that the combined levers are heavier and have the unnecessary side-ways freedom of movement. But that's fine if it doesn't bother you. If you find out that this is unnerving then you may switch to regular brake levers.

The day you want to change the tape, you may acquire a pair of regular brake levers and put them on the bar. 

Edit: No, you can't keep the shifting cables running from the bar just through the friction shifters. You'll need to remove them and rethread them directly through the friction shifters.