The simplest way to modify your CS:GO crosshair is by CSGO Skins way of this Steam workshop map crashz' Crosshair Generator. Then it ought to be on your workshop map list in your private offline servers.
Jump into CS:GO, click "Find a match," and pick the "Workshop" tab next to "Common." From the record of maps, pick the Crosshair generator map and then click the green "Start local server" button beneath the map option. Your very own private server should load and you should spawn in an area with menus onto a wall before you. These menus can allow you to correct your current crosshair. Making mistakes for this crosshair generator is fine too, since you can always reset your crosshair with the control panel on the right.
To start, select a Crosshair style that you like and are able to target well with. A few of the styles are static, some are dynamic, and some are a mix of both. Pick whatever is your preference. Depending on the style you selected, numerous secondary menus must be accessible. Examples of these menus comprise crosshair size, thickness, gap, outline, alpha, and dot. Upon selecting a submenu, more options on a panel should appear. These numbers change the number values in the developers' games console, so that you don't have to manually type in the controls for every tiny crosshair adjustment. For players who are more sophisticated with the keyboard, a Complete list of these commands can be found here,
Now that you're done changing the smaller parts of your crosshair, you could even change the color to whichever works better for you personally. It is strongly suggested that you utilize a white crosshair using a black outline, so you don't ever lose your crosshair while aiming at differently colored walls. However, if you are always consciously focused in your crosshair, you won't need to worry about that problem in competitive games.