It's easy: buy two dolls, one Ken, one Barbie. Switch their clothes.

From there, there are ALL KINDS of options:

• change their names and preferred pronouns
  • Ken ⇒ Kendra (she/her)
  • Barbie ⇒ Barney (he/him)
• give Barney a beard shadow (use a brown marker)
• have Barney wear a binder (unfortunately this isn't as easy as in real life — you might have to sand his chest down)
• give Kendra top surgery (with a hot glue gun)
• give Barney bottom surgery (with a hot glue gun) (Kendra already had bottom surgery done at the factory, long before she came out of the closet)