I have done this many times. I even once used about 5 grams of roach weed, split the batch with a friend and got really baked.

You will need one of those pyrex measuring cups. Something safe for the oven.
10 grams ground up(the amount you put in is up to you, really should be no less then 5grams)
a box of brownie mix
vegetable
and an egg, two if you like it cake like.

measure 1/2 a cup of vegetable oil into the measuring cup (OVEN SAFE). mix the ground weed into the oil so it all is saturated. Put it in oven that is preheated for 350 degrees. Bake it for about 20 minutes. Stir it once or twice for good measure while its cooking.

when its done carefully remove it from the oven. Make the brownies as instructed by the box. You can either strain the thc-laden oil into the mix or just dump it in. I recommend straining it.

cook those brownies and then enjoy. your gonna want to eat around 1.5-2grams of pot, so do the division to determine serving size. Effects can last up to 8 hours, more if you are using really good sensi.

I find that you dont have to use as much weed if you replace the vegetable oil with butter. Or if you use the same amount its much more potent.