How do I get my girlfriend to make me a sandwich?

You tell her you love her. Get up and make one yourself

Ask her politely and with the promise you'll do something for her.

Ask her., if she tells you no, move on from that lazy thing. It only gets worse.

Start by demonstrating that you can take care of yourself and treat her like a partner (not a domestic servant).

Make her favourite sandwich for her for the next five days.  Then on the sixth day ask if she would make a sandwich for you.  That normally works well.

Get a big loaf of bread, then spread mayonnaise all over yourself.

buy her this sandwich grill

https://www.amazon.com/Hamilton-Beach-25410-Sandwi...

and she can use it to make u a sandwich

Wear nothing but boxers and a white, stained, tank-top tee, drink a can of beer while you're watching a football game, make sure you haven't shaved or showered in about 2-3 days, and yell out "Fix me a sammich, woman!". That's what works for me. 

You have to get a girlfriend first ☜(ﾟヮﾟ☜)