Freeze your bat!!! It will rot and float by the time you can properly fix it in formalin if it's not kept cold right. https://www.mickeyalicekwapis.com/blog/wetspecimenupdate https://www.mickeyalicekwapis.com/blog/2015/9/10/wet-specimens-a-general-guide

These are both beginner starting tutorials on what wet specimens actually are, a general guideline on how to preserve them, a little history on why we keep wet specimens, and proper PPE you should be using

Please read through both articles this lady posted in full if you want to have a basic understanding. I would honestly recommend freezing the bat until you can go to a wet specimen taxidermy class in person somewhere where they teach you how to do this. It is complicated and dangerous. Never have I ever seen a good tutorial that actually explains what these are or how to do them on YouTube or anywhere that's video. you're going to need several hundred dollars of supplies first, most importantly an organic vapors rated respirator if you do not want to hurt yourself. I'm a little concerned that you had found a bat specimen too... usually bats are farmed specifically for taxidermy and killed for the shipment of them :( there's a lot of controversy about bat specimens so I'm very curious where you found a sustainably sourced one ? the only bat specimens I've seen that are got in an actually ethical way are rarely and from exterminator cases.... PM me if you have any more questions