Advanced (for me) Query help

SQL 2000 - I have two tables that I need to manipulate some data between.
I need to develop an sql script that will do the following:

Update the ManagerID field in Table1 with the Supervisors ID from Table1 based on the persons defined Supervisor in Table2
All ManagerID values in Table1 start out at 0

Example for after script has been run:
John Doe's ManagerID in Table1 should be 50
Dave Smith's ManagerID in Table1 should be 51
Mary Jane's ManagerID in Table1 should be 52


Example Tables before script is run:

Table1
ID UserID Last_Name First_Name ManagerID
1 20 Doe John 0
2 21 Smith Dave 0
3 22 Jane Mary 0
50 Maint Sup Smith Bob 0
51 Prod Sup Wilson Kevin 0
52 Unit Oper Peters Gary 0

Table2
UserID Last_Name First_Name Supervisor
20 Doe John Maint Sup
21 Smith Dave Prod Sup
22 Jane Mary Unit Oper

Looks like a classroom excercise - in which case you shoudn't be cheating.[:#]

lol..I promise I'm not. This is for work and I've had no formal training.[]
My days in school are long gone, now it's all ojt.

Fair enough. But this is pretty basic query stuff - execute an UPDATE query that includes a FROM clause with a JOIN.
See if you can write a SELECT query that returns the results for the key columns next to eachother. Now you already have the FROM and JOIN parts, and all you have to do is rewrite the SELECT part into UPDATE and SET clauses.

I was able to come up with this with some excellent help from some more experienced users.
The real tables are:
Table1 = TLMUser
Table2 = Supervisor_Tmp
UPDATE t1

SET t1.ManagerID = t1_2.ID

FROM TLMUser t1

JOIN Supervisor_Tmp t2 ON t1.UserID = t2.UserID

JOIN TLMUser t1_2 ON t2.Supervisor = t1_2.UserID
The only problem I have now is that there are 47 rows that should get updated but only 33 get updated. The 33 are perfect by the way, I'm trying to figure out why the others are being ignored.
It was suggested that the UserID from TLMUser may not match the UserID from Supervisor_Tmp. I've looked and compared line by line and they do appear to match. Trying to track down what could be different between the two.
Any ideas would be greatly appreciated.

The problem was in fact that the UserID from TLMUser may not match the UserID from Supervisor_Tmp I had to clean up the data before importing it again. After everything matched, the script above did exactly what it was supposed to do.
Thanks to everyone that reponded, hopefully this may help someone else with a similar issue.