how to explicit values to be inserted into the identity column of a table.

Two conditions are needed:

1) set identity insert to ON:

SET IDENTITY_INSERT db1.dbo.table1 ON

2) specify column list, for example:
INSERT INTO
DB1.dbo.TABLE (column1, column2, column3)
SELECT column1, column2, column3
FROM dbo.TABLE1
dbo.TABLE2 ON dbo.TABLE1.column = dbo.TABLE2.column