Hiya, is your 3d bike standing or like the Harley in my pics?
If its standing, because its so top heavy youll need to attach something like a piece of GP or RKT to the top of a dowel or it will just fall over, then you should model out from that point, youll need to make your wheels early and let them dry or they go oval shaped! Same with the handles so they dont droop!
Once you have these three basic parts; body on dowel, wheels and handles you can attach them together with thick royal icing and build the detail up using a photo for reference.
For mine I got a side on pic of the bike and made a black silhouette shape then worked up using a pic as reference, this had to travel a long way so I had to make it flat but I really wanted it as 3D as possible.
HTH good luck- would love to see it when youre done!