Creating grid in Adobe Illustrator

To create a grid in Adobe Illustrator, you might have several ways to go about it.

First, one can be to use the default grid which can be chosen from the view Tab. This however is very constrained and limited. 

If you want to create a desired grid according to your specification, then follow these steps.

Launch Illustrator and create a New document.

Create a rectangle as shown below

Press M to select the rectangle tool or go to the tools panel and select the rectangle tool.

Move to the Object Tab, then hover over Path and then click Split Into Grid.

Use the arrow in the pop-up menu to select the number of rows or columns desired. Select the gutter too as desired. Make sure to check the preview button to see what the grid looks like as you make your changes. when you are done, click ok. 

The modular grid below has Rows - 3, Column - 3, and gutter for both rows and columns 12pt.

Remember, Illustrator split the initail rectangle create into broken down rectangles forming the grid and each is indvidual. You can select all and group so it does't destruct as work is on going on the artboard.