If I leave my job and spend my 401k, how do I file the 10% early withdrawl on my taxes? Also do I need to file the 20% withheld for me to pay the following year's taxes?

[Critter-3]

In January a form 1099-R for the entire distribution will be mailed to you and    that must be reported on the tax return ... and the taxes that were withheld will be credited against the total taxes owed which will include the penalty.  They withheld 20% however that may not be enough ... 10% is for the penalty and 10% for the fed taxes however the distribution plus your other income will most likely put you in a higher tax bracket ... then your state may also tax the distribution ... so you may need to do some calculating and make an estimated payment before the end of the year.  

[VolvoGirl]

Turbo Tax will automatically calculate the 10% Early Withdrawal Penalty and enter it.

You don't actually pay the tax or 10% penalty (you pay a 10% early withdrawal penalty if you are under 59 ½).  You have taxes withheld like from your paycheck. You still have to enter the whole gross original amount (before taxes were withheld) with your other income to figure out the total tax (and it may put you into a higher tax bracket) and then the withholding is subtracted from the total tax to figure your refund or tax due. The gross amount shows up,on 1040 line 4a/4c and the taxable amount on 4b/4d.  The withholding will show up on 1040 line 17.

It will break out and show the 10% penalty separately on your return, (Schedule 2 line 6 which goes to 1040 line 15)

Then you get credit for all the withholding taken out on 1040 line 17.