Is it possible to turn into something that you have no idea what it’s like to be? I was thinking of different things I would attempt to transform myself into and someone mentioned I should try the opposite sex. Then I got curious what it would be like to be a woman which is then when I realized that … if my brain has no idea what it’s like, how can it reproduce it in a dream? Same goes if I wanted to be a tree or a bear.
It’s not as hard as you might think. For the gender thing, picture what would be different. You’d probably be shorter, lighter, with smaller hands and feet, thinner shoulders. You’d have wider hips and your center of weight would be off (which may take some getting used to). Really, if you feel you can picture all this pretty well, then you’ve got a pretty good guess of what it would feel like.
And that’s what it is, really. An educated guess based on what you- and your brain- knows about biology. Considering it’s a dream, it may or may not be accurate. But in a dream, your brain can be pretty damn accurate about assuming other forms (or at least, so you think). Human forms are especially easy.
As far as animals go, it isn’t much different. Switch up some bone structure, add extra bones here and there, a more pronounced spine, probably quadrupedal and probably with a longer snout in place of your face. Again, you might not no for sure, but you’ll surprise yourself in how good an idea you can come up with how it feels if you actually try.
Trees and fire are different- since they aren’t living things (in the sense that they don’t have organs, a brian, or whatnot-trees are certainly made of living cells) there really is no way to truely experience being a tree, so you’ll end up with whatever you- or your brain- guesses. I felt branches and leaves on them, but not much else, except the change of the seasons. With fire, i’d guess you’d feel a wave flow through you as little bits of yourself licked off and became embers.
This probably sounds like incoherent rambling, but remember the jist of it: it’s all in your head, and your brain is smarter than you give it credit for. It can be surprisingly good at filling in the blanks at times, and while you’ll never know how accurate it is for sure, you will hardly care. The important thing to remember is that believing you can do it is the most important aspect of success in LDing- doubt yourself and you might find transformation harder than it should be (trust me on this, it sucks only being able to transform halfway!), but get it right and i guerentee you’ll have fun!
Credentials: I’ve been all of the above- multiple times (in fact i’d have to say i’m probably male in about 50% of my dreams and female in the other 50).
It would probably be easier to more experience you have with interacting with said form. I bet you see women every day. Study their shape, imagine how you would look and sound as a woman. The same with animals, play with pets, imagine how it would be like to walk naturally on all fours, to have whiskers, a tail etc. If you put thought into it during the day, it will be easier to use it during the night.
Have you ever had non-lucid dreams in which you were somebody/something else? Try to remember how that felt. And even if your first transformation isn’t perfect, perhaps you become half-you and half-other, try to explore it as much as possible. And trees might be very different, but if you stand still and just tell yourself “I am a tree, I have branches, leaves and roots”, and really picture yourself as a tree in your head, I see no reason why it would be impossible. Maybe you should pick a certain kind of tree, just to have clearer image in your mind. Oaks, pines, sycamores, gingko, linden trees… choose your favorite!
I’ve transformed several times into a wolf, I’ve also tried to transform into a bird (succeeded in growing wings at least) and into my aunt.
You DO have an idea. You can’t tell if your ideas are accurate, but your brain doesn’t care. It can define a woman. I’m no expert on transformation, but I have had the head of a cartoon crocodile once, although in the mirror. I had four arms last week, and have had extra fingers many times. It is weird when you feel them, but it does happen. Even though I couldn’t imagine what it would feel like, WHERE I would feel the extra digit. I can say this though. You can become anything. Don’t worry about what it is like. If you can SEE it, then sensations will be filled in as well as your brain can improvise. Try using a mirror to get the image.
i tend to agree with krakatoa. things like this tend to go like: you say ‘i don’t know how it would be, i have no idea,’ then you experience it and you say ‘i did know! that must be what it’s like! i guess i already figured that, just never vocalized it/called it to attention’ (not to put words in your mouth 
) anyhow, be open to the idea that you already likely have some rudimentary idea of how things would be and let your mind fill in the blank spots
Well, you know what you could try? You could attempt at summoning some sort of fairy or something, haha, and ask it to transform your body FOR you. Get what I mean? That way your not actually doing it yourself, so you can dodge feeling restricted because you don’t actually know what it would be like.
nice technique 
Turning into a shark swimming in the ocean would be cool.
I read/heard you have to be confident that you can.
I’d want to turn myself into a Nav’i and roam around Pandora. Expecially since lucid dreams are “vivid” and are extrememly “realistic” and “detailed”.
I’m having a hard time remembering my transformations, so I can’t really vouch for how detailed our SC can make things. I think there was a dream I had once a few years ago…but what was it? Something about transforming…either into an animal or having webbed feet or something…
Ah! But there was and instance where I was flying with wings protruding from my back–and I even saw my own shadow.
And I remember having the power to change my voice so I’d sound like a dude. Weird feeling. Someone was looking at me funny, on account that it was the only thing about myself that I transformed!
Imagination can fill in a lot of things.
When I LD of being a woman, just as Win Laik Pya says, it’s easy to imagine. My other favourite is becoming a dog (but I’ll try being just about anything on four legs), which again is something that my imagination can fill in a lot of things for me. I have trouble turning myself into a bird in my LD’s though for some reason, but can fly.