SUMMER IS a good excuse for casual entertaining. Sundresses and sandals, blue jeans and knit shirts should be the only dress requirements for the summer dog days. And the menu should be casual, too. This is a good time to bring out your best Tex-Mex recipes, your favorite sangria potion and those dusty paper flowers you brought back from Mexico.

There's no need to worry about appetizers; guacamole and corn chips or bean dip with tortilla chips are perfect with beer, sangria or margaritas.

For 12 people, chicken enchiladas, chiles rellenos souffle', barbecued bean casserole and a marinated zucchini, avocado and green pepper salad would be a filling menu that can be prepared ahead. Buy an album of Mexican music and you're on your way to a fiesta.

GOOD AND CHUNKY GUACAMOLE 4 large soft avocados Juice of 2 limes 4-ounce can of chopped green chiles 1 large red ripe tomato, skinned and seeded 4 green onions, minced

Peel the avocados and mash them with a fork. Add the lime juice and stir well. Chop the green chiles a little finer and chop the tomato into tiny cubes. Stir both into the avocado. Mince the onion and stir it in. Serve with lots of corn chips.

SUPER BEAN DIP 3 4-ounce cans prepared bean dip 8 ounces cheddar cheese, grated Jalapeno peppers in escabeche (pickled hot peppers)

Spoon the bean dip into an ovenproof pottery bowl. Stir in the cheese. Cut the peppers in rings and use to garnish the top of the dip. Heat in a 350-degree oven for about 15 minutes or a microwave oven for about 3 minutes. Serve with tortilla chips.

SOUR CREAM CHICKEN ENCHILADAS (Makes 2 dozen) 3- to 4-pound chicken, poached and shredded 4-ounce can of diced green chiles 1 cup chopped black olives 1 cup slivered almonds 1/2 cup raisins 1 cup salsa verde (available in Mexican sections of groceries) 2 quarts sour cream 2 dozen corn tortillas 1/2 cup vegetable oil 1 pound colby or longhorn cheese, shredded$51/2 cup minced green onion

Have the chicken chopped or shredded in small pieces. Place in a large bowl with the chiles, olives, almonds, raisins and salsa verde. Toss well. Add approximately 1 quart of the sour cream to bind the ingredients. Set aside.

Fry the tortillas until they are soft in about 1/2 cup of vegetable oil. Fill each tortilla with a large spoonful of the chicken mixture. Add a tablespoon of shredded cheese and roll up. Have 2 rectangular baking dishes ready. Spread about 1/2 cup sour cream in the bottom of each dish. Place the enchiladas in the baking dishes, seam side down. Top with the remaining sour cream and then the remaining shredded cheese. At serving time, bake in a 350-degree oven for about 15 minutes, long enough to warm through and melt the cheese. Garnish with the minced green onion.

LOUISA'S CHILES RELLENOS SOUFFLE (12 servings) 2 7-ounce cans of green chiles 3 eggs 3 cups milk 1 cup flour 1 teaspoon baking powder 1 pound grated cheddar cheese 1 teaspoon salt

Finely chop the chiles. Beat the eggs and add the milk to them. Put the flour, baking powder, cheese, salt and chiles in a large bowl. Add the milk mixture and stir together. Don't overmix. Grease a large casserole or souffle' dish and pour in the mixture. Bake at 350 degrees for about 45 minutes. The time will depend on the shape of the dish. The souffle' is done when it is puffed. This is not a true souffle' and will fall when taken from the oven. To serve, spoon out. Any leftovers may be frozen.

BARBECUED FRIJOLES (12 to 16 servings) 1 1/2 pounds pinto beans, soaked overnight 1 tablespoon salt 2 pounds fatty pork (shoulder) 2 large onions, diced 4 cloves garlic, mashed 2 teaspoons cumin 1 teaspoon coriander 3 tablespoons chili powder 1 1/2 cups tomato sauce

Wash the beans and soak them overnight. Simmer for an hour in the water they soaked in, adding more as needed. Add the salt and continue simmering. Cut the pork in 1/4-inch dice. Saute' it in a large skillet. Add the diced onions and garlic after the meat has lost all tinge of pink. Saute' until everything is lightly browned. Add this to the beans along with the remaining ingredients. Simmer for a total of about 3 hours or until the beans are tender and mealy and the sauce is thick and rich.

FLAN WITH WHIPPED CREAM AND FRUIT (12 servings) 1 1/2 cups sugar 1/4 cup water 1 quart milk 1 tablespoon vanilla 1 cinnamon stick 8 eggs 1/2 teaspoon salt 1 pint strawberries 1 ripe papaya 1 cup diced canned pineapple 2 cups diced fresh peaches 2 cups heavy cream, whipped

Place 1 cup of the sugar in a heavy pan with the 1/4 cup water. Place over high heat and cook until the sugar becomes a golden brown color. Pour the caramelized sugar immediately into a 12-cup ring mold. Using oven mits, tilt the mold to cover the bottom with caramelized sugar. You must work quickly or it will become solid. Set aside.

In another large pan, heat the milk with the vanilla and the cinnamon stick. While it is simmering, beat the eggs with the salt and the remaining half cup of sugar. Remove the cinnamon stick from the sugar and whisk the hot milk into the eggs. Pour into the ring mold. Place the mold in a larger baking pan and fill the baking pan with boiling water so that it comes halfway up the side of the mold. Place in a 300-degree oven for 1 1/2 hours or until the custard is firm. Cover and chill in the refrigerator for two or three days.

At serving time, run a small knife around the edges of the ring mold. Invert on a large round platter. The platter should be deep enough to hold the melted caramelized sugar. Surround with the fruit. Whip the cream and place it in the center of the flan. Serve each guest a little flan, a spoonful of fruit and a dollop of cream.

CHEAP AND EASY SANGRIA FOR A CROWD 2 12-ounce cans frozen lemonade (undiluted) 2 quarts inexpensive red wine (a jug type burgundy) 2 lemons, sliced thin 2 oranges, sliced thin 3/4 to 1 1/2 cups sugar (depends on your taste and the wine)

Just combine everything except the sugar. Taste. Add sugar only if you feel it needs it and then add it cautiously. Pour over ice.

MARGARITA FRAPPE 1 1/3 cup fresh squeezed lime juice 1/3 cup cointreau 1 cup tequila 3 cups crushed ice Kosher salt

Place the lime juice, cointreau and tequila in a blender and add the ice. Turn the motor on to the highest speed and let it run until the whole is smooth. Add more ice if necessary to make a very icy drink. Use a lime rind to wet the edges of several champagne glasses and dip in the salt. Fill with margaritas. This is not an authentic margarita, but the taste is right and the chill is welcome on a hot evening.