Pick her up and place her gently in the litter box.  Press down on her
lower back to show her the sitting position then carefully take one
paw and show her how to scratch the litter.  Be sure the box is in a
quiet place where she won't be disturbed.  You didn't say how old the
kitten is.  Is the box easy for her to get in?

Do you only have one litter box?  Some cats will not use the same box for
both feces and urine (and some will use the same box for both part of the
time but not all of the time).  I suggest you get a second box and start out
with them side-by-side.

MaryL

Lots of good suggestions so far. Here's more...

Does she always go in the same spot? Put a very shallow (no more than an
inch high) container there (a large storage box lid might do - it should
be at least as long and wide as the cat's body) with a very small amount
of litter in it.
If she uses that, she may not like something about the litterbox. Change
it.
If she doesn't, she may not like the litter. Try another kind (or even
just newspaper in the beginning).

Some cats prefer hooded (covered) boxes. Some hate them.

Some cats prefer scoopable litter because of the finer grains. Some cats
hate the smell - try unscented.

You ARE scooping the box daily, right? Try twice daily. She may not like
to step in her own excrement.

You didn't use any type of scented or overly strong cleaning solutions
on the box when you cleaned it, did you?

IME, using the same type of box and litter as what she had before you
got her is helpful.

Praise, praise, praise. Never yell if she makes a mistake - she will
only learn to hide it from you by going in corners or under the bed. Sop
or pick it up, put it and her in the box, and PRAISE. High-pitched happy
voice, pets, and leave her be. If you put her in the box when you are
angry, she thinks it is a bad place. Don't do that.

That's all that comes to mind ATM...

Let us know how it goes.

Hi Darren,

You got really really great advice so far... It always depend on the source
of the problem as to what solution you will choose.
I will share my own experience with you. My younger cat took a long time to
go in the litter box for every "business" she had to do. I have enough boxes
for 4 cats (one more than the number of cats), in quiet corners, cleaned
once or twice a day depending on how much they go... etc etc.

Yet, she was going in my office. So I acted with her a little bit like we do
with a 2 years old toddler when we potty train them. When I was home, I
would pick her up and put her in the litter box every hour or so or more if
she was agitated... you have to observe her behaviour a lot a bit like you
do with kids and remind them.. "do you have to pee?").

Sometimes she'd do something, sometimes she'd just step out... The business
"out of hte litter box" got a little less cause I didn't give her a chance
to do it other places. Then, when she did it in the litter box many times
(with praise and scratch scratch and pets pets afterwards) I started to
reduce the number of times I would "remind her". And when I did (every 2
hours or so) I didn't pick her up anymore. I encouraged her to follow me,
until she'd get to the litter box and I would encourage her to get in it
etc...
And I went like that until I noticed she was going by herself always. It
took patience and hard work but within a week or two, the problem was
solved.

Good luck,
Mia :)