Creating a wireless ad-hoc network on linux is a fairly simple task. The only problem might be with the wifi device drivers as some may not support this feature. Make sure the normal wifi connection ( connection to an access point) works. If you have two Machines A and B this is what you would do.

Step 1: Change wifi interface configuration to ad-hoc

On both machines:

iwconfig wlan0 mode Ad-hoc
iwconfig wlan0 essid MyWifi

You can also set other options such as encrypting with the iwconfig command. Check man iwconfig

In my case, the mode change command gives a resource busy error if wlan0 interface is up. That is, if you can see wlan0 when you do ifconfig, then your wifi interface is up.  Here is what I get:

Error for wireless request "Set Mode" (8B06) : SET failed on device wlan0 ; Device or resource busy.

The only way I found around this problem is to do the iwconfig commands before I configure the wifi interface with ifconfig.

Step 2: Set the IP addresses

On Machine A:

ifconfig wlan0 192.168.1.1 netmask 255.255.255.0

On Machine B:

ifconfig wlan0 192.168.1.2 netmask 255.255.255.0

You can choose different IP addresses but make sure you have the right subnet mask.

You should be set. You can now ping.