First, please notice that Excel has documentation all along the way. Just type "=pmt(". As soon as you type the left parenthesis, you should get a picture of available parameters. The same with many others. Try "=fv(". It's great stuff and should help you on your way.
The deterimination of what to do with mismatched interest crediting and payments MUST be specified in the problem statement or you should have a very good argument if you get it wrong on an exam. Generally, unless you are an insurance company, interest rates are reported as the rather useless NOMINAL Annual rate. That's all fine, but you must know about compounding in order to get the right answer. It also helps to know a little about common practices. For example, auto loans often report nominal annual and take payments monthly. For auto loans, simply dividing the nominal annual rate by 12 will do. If this sort of thing is not given in the problem statement, you state it clearly before proceeding. This will give you power when you argue the point with the grad student who marked it wrong.
In your case, enter simply "=pmt(0.06/12,48,25000,0,0)"
0.06/12 is the monthly interest.
48 is the number of months. Notice that this is 4*12 = 48
25000 is the beginning balance
0 is the final balance. This is good for "salvage" questions. If there is no salvage value, or if the loan is completely paid off, you can use "0" or simply omit the value.
0 is an option switch. Uusually, you can forget about it as a "0" or nothing means payments are at the end of each period. sometimes you want payments at the beginning of the period and this will require a "1" in the fifth paramenter.