Right-click your game shortcut and select Properties.
Click the Shortcut tab and look for the Target field.
Click at the very end of the text (after the closing quotation mark), press Space once, and paste this exact text:
text
+set com_hunkMegs 1024 +set com_zoneMegs 128 +set com_soundMegs 160
Click Apply and then OK.
Your final Target box should look like this:
"C:\...\etl.exe" +set com_hunkMegs 1024 +set com_zoneMegs 128 +set com_soundMegs 160