1) Install Apatar

2) Download MSSQL-to-PostgreSQL DataMap at http://www.apatarforge.org/files/business-productivity/mssql-to-postgresql/download.html. You can use "Try it/Edit it" link to open DataMap with Apatar application.

3) In Apatar's work panel, you will see two connectors and Transform module.

4) Double-click on the MS SQL connector and fill in necessary authorization details.
4.1 Then choose a table you would like to extract data from.

5) Double-click on the PostgreSQL connector, fill in authorization details.
5.1 Then choose a table you would like to transfer data to.

6) Double click on the Transform module.
6.1 On the left, you will see the list of MS SQL source data fields.
6.2 On the right, you will see the list of PostgreSQL target fields.

7) Drag-and-drop to Transform's work panel the fields you would like to extract data from, and the fields you'd like to transfer data to.
7.1 Link (map) the left fields and the right fields together.
7.2 Click OK. Transform window will close.

8) Now all you have to do is start data migration. Choose File menu -> Run

You will see the progress window, and then the preview window with the migrated data will pop up.