The safest way is to send your cel to a 
professional cel restoration company. You can 
find it by doing an archive search here or ask 
for the URL someone is bound to know.

Beyond that:

The level of risk involved in doing it yourself 
depends on a couple of factors:

A.) Do you really want the sketch? If you do, 
then you will most certainly damage it to some 
degree no matter what you do, especially if it is 
stuck "deeply". If it is stuck "lightly" you can 
sometimes get away with just very lightly pulling 
it off, simple as that. You can tell because it 
won't take much effort at all and you won't get 
any centered resistance, but just some coming 
from around the edges. This is very, very light 
tugging I remind you. Otherwise, you may want 
to "freeze it" in a freezer for at least a few 
hours making sure that the cel lays flat. I have 
tried this with inexpensive cels before and I 
have sometimes been able to remove the cel from 
the sketch with insignificant or no paint loss. 
Other times I have failed, because I was in a 
rush, or it simply was too stuck in the first 
place and the freezing method did not work. So 
either way don't just believe it will work 100%, 
more like there is a chance that it may work. You 
are taking a chance! Paint loss to the cel is the 
most important thing for you to avoid at all 
costs if possible, so please remember that.

B.) If you don't care about the sketch so much, 
or if you really want to get it off at all costs 
(or because you tried the freezer approach and it 
didn't work), Then you can "wash it off" very 
gently with water. You can wet the back side of 
the sketch, actually soak it by lightly running 
or spraying water on it (cel) until the paper 
begins to dissolve, and oh so carefully peel the 
paper off the places that are easiest to peel 
first saving the really hard core areas for 
later. If the paper does not peel easily off even 
after you have done soaking approach, I would 
just leave it alone if it doesn't show through to 
the other side, because if you are too rough you 
will get paint loss and "punch holes" in your cel 
image by ripping the paper off that is too stuck.
And you will take a chance of possibly dissolving 
some of your paint away with too much water, so 
again be careful and don't over do it. You will 
have to dry your cel with a clean soft cloth that 
doesn't shed fibers or scratch the acetate or rub 
the paint off. Maybe just blot dry it or hang it 
up for awhile and then in a few minutes blot dry 
it with the cloth. You will have water spots on 
the acetate and you will want to plot those away! 
If you have clear clean bottled water, rather 
than tap, I would you it. But, I know a lot of 
people may moan about what I have just told you.
In any event use your best judgement and common 
sense if you decide to risk it yourself. Be 
GENTLE and don't step off too much into la-la 
land of the unknown.  ^-^