You open the writing from your LSAC account. You login, select LSAT writing, and then scroll to the bottom and it will say launch LSAT writing. From there it will prompt you to either download the proctor software, or just launch the test if you already have it downloaded. There's a tutorial called "Get Acquainted with LSAT Writing" that allows you to practice the sign in and test out the proctor software on your computer before you launch the actual LSAT Writing too, if you want to see what it will look like. That's also available on the same page on your LSAC account