Like any other SPICE . directive: To the schematic of your sim, anywhere it doesn't get in the way!
If you are coming from another SPICE simulator that uses a separate pane for directives, well LTspice doesn't do that. Use the [.op] button at the far right of the toolbar to enter directives, SPICElines and even inline models in the dialog textbox it gives you, typing Ctrl-Enter if you need a new line for multi-line directives etc. then when you click [Ok] or hit Enter, you must then place the resulting text block on the schematic. You can right click an existing text block to reopen it for editing, and then can also toggle it between netlisted as SPICE Directive (it does stuff) and Comment (its disabled).
Also, if a sim has associated custom or 3rd party models or symbols, its a good idea to make a sub-folder in your LTspice working folder, named for the project, and put all the required files + the sim in it to keep them together.
Here's the quick test jig I made to confirm the T.I. model was compatible with LTspice, as an example.
N.B. for anyone else following this, you'll need to grab the T.I. 741 model from their website.
P.S. *PLEASE* be careful when quoting someone, when you type text into a reply and accidentally get it in between the BBcode quote tags, it posts looking like something *THEY* said. You get a free *noob* pass this time, but do try to avoid putting words in other peoples mouths as it results in misunderstandings, disgruntlement and grumbling. --
I am not a moderator here, nor do I play one on TV!