help for novice with bivy

Generally on the inside of the bivy is where the pad goes. It can go outside if you want, but I don't know why you would if it fits inside unless it was wet.
When it rains and you set the tarp up, unroll the bivy with the waterproof side down, put your sleeping gear in and get inside if you want.

It sounds like I don't need an extra ground sheet and just rely on the waterproofed bottom. Am I correct?

Many people who want the lightest weight and smallest packed size just use the bivy bottom. If it's silnylon, then water will be forced throught the fabric if you lay on waterlogged ground, so choose the site carefully.

Many people who want more convenience use a 3×7' polycro / window insulation film, tyvec, or 2-3mm painters plastic groundsheet. This gives waterproofness in waterlogged spongy areas, and a bigger "clean" area to move around on in muddy weather.

does everybody buy equipment made from silnylon 2nds or something? I've had puddles under my floor with silnylon 1sts in my shelters, that I have sewn myself, but I have never had a problem with the floors yet. We are talking from the soaked ground from the snowmelt along rivers, the rain forests in the Olympics and the rainy cascades… I guess that's another reason for me to keep sewing my own equipment, that way I know what the heck I'm getting…

The only problems I have had with tent floors leaking are from cheap shelters, like the ones walmart sells.

I may be the odd one out here, but I put my sleeping pad on the outside of my bivy. Typical setup for me is a 1/8" CCF pad first to protect the bivy bottom. Next is a torso sized pad and then my bivy on top…I don't know if this is correct or not, but it works for me. :)

>Do sleeping pads go inside or outside of a bivy? Also what do you do if the ground is wet or it's raining (assume that I figured out how to erect the tarp)?

It depends on your preference and your pad. Some of the inflatable ones (eg MB) recommend you don't place them directly on the ground, because of risk of puncture. Also if the inflatable is packed away in your bag you can keep it clean and dry.

I mostly use a foam pad and have it outside my bivy, to protect the cuben bottom of my bivy from rough ground. Also if it's been a wet day, the pad will be wet when it comes out of my pack (and if it's on the outside of my pack, muddy too), so I don't want it inside my bivy

If the ground is wet and you've got a silnylon bottomed bivy, as some have mentioned you *might* have issues with water getting forced through at pressure points. It depends on the weight of the silnylon. I doubt it would be a major problem unless you're using no pad at all. I haven't had this problem on wet ground when sleeping on a foam pad inside my silnylon-bottomed Tarptent Rainbow. Tarptent don't list what weight silnylon their tent bottoms use though, it might be thicker than the 1.35 silnylon used on MLD bivy's (for example).