You will need:

• Equal parts crushed ice or ice chips (depending on how heavy-duty your blender is!) and de-seeded watermelon. Maybe you can get one of these and avoid the hassle of doing it yourself. Pulse until pureed.
• Tequila and Triple sec in proportions of 3:1
• Lime juice in approximately the same amount as tequila, depending on taste
• Lime wedges for serving (optional)
• Salt and sugar for glass rims (optional)

It’s best to prepare this in a cocktail shaker to really get everything to mix well before serving. You may prepare a mixture of sugar and salt for the rims of your glasses. Just before pouring your cocktail mix into your glass, place a small amount of salt or sugar into a saucer. Rub the edge of your margarita glasses with a lime wedge to moisten. Lightly dip the rim of the glass into the saucer; tap off excess salt or sugar. Garnish with a lemon wedge.

Since you’ve already used ice in your watermelon mixture there is no need to top up with ice.