Depends a bit on mods, but the two big ones are either kPlug (which is optional and not automatically enabled in downloads but is debatably worth installing, just as long as you go through all the precautions, it clashes with a lot of things), or the uncensor mod which I think is enabled by default.
First thing's first: go to options on the main page even before you create a character and find either the uncensor mod (top right corner is a black mod options button, you may have to further click on 'advanced mods'). If you have kPlug then you installed it yourself and can read the instructions that come with the mod, but basically rather than create a futa character it can flatout make every female character one and you don't have to modify any directly.
Everything from here assumes uncensor mod as it's easier for me:
1. Download the drag and drop mod I linked a page or two ago. While I won't say whether creating a male or female based futa is bad or wrong, once you have a character you like the look of, this game is kinda limited in terms of having one model take part in multiple parts, so being able to easily make a male and a female card of the same character does mean that it can be used for penetrating others or being penetrated. Basically once you make a card you like the look of in say, the female editor, with the drag and drop mod you can open the male creator, drop the female card into the game window, and it'll automatically turn into the card you dropped.
2. Again, assuming uncensor mod, to turn a female into a futa click on the second tab of the creator that looks like a body. In the general options, if you look down you should see three options: body, penis, and balls. If you're playing in the female creator, just add penis/optional balls and voila. If you're playing in the male creator, there is no easy way to add a vagina that doesn't mess with things (which may or may not be a concern as again males can't be penetrated, but as
alex2011 noted guys with breasts may not look like what you want). For guys then, click on the 'body' setting and select one of the females, this will keep your character's current face but will give them a female body based on what you pick. If you liked the guy body and just wanted a vagina then you'll probably have to mess with sliders a bunch.
3. Unrelated kinda, but in the male creator on the <3 tab I believe, there is an option to "become a trap". This doesn't strictly make you a futa, but it does modify your character's movement stuff. If you wanted to play as a futa in the story (or even just a femboi) and wanted to run like a girl, you check this box and mess with movement options.