Hi Chandra,
Making slot in patch antenna is simple. Here are the steps
1) Draw a rectangle equal to dimension of patch
2) Draw a rectangle of size equal to slot above the patch.
3) Select the two structures
4) Right click to get "edit - Boolean - substract as shown in screen shot
Now window pops up and mention antenna as "blank parts" and slot as "tool parts". Uncheck "clone tool objects before operation. (refer the image)