How do I screen share between iMac and Macbook?

In the laptop, visit System Preferences > Sharing and enable Screen Sharing. Take note of the computer name in that panel. You will reference it on the iMac as hostname.local where hostname is your laptop computer name.

On the iMac, drag and drop /System/Library/CoreServices/Applications/Screen Sharing.app to your Dock. Power on the laptop and give it time to boot up to the login screen. On the iMac, click the Screen Sharing Dock item and enter the laptop hostname as mentioned in the first paragraph. The laptop screen should appear on your iMac, and you can drag/drop files between machines in this interface. You cannot, however, display the laptop as a full-screen entity.

Everything VikingOSX says.

Also, on the laptop, check System Preferences -> Sharing -> Screen Sharing to see that "Allow Access for" is set so that you are allowed to Screen Share with your laptop.

You can also use

Finder -> Go (menu) -> Connect to server -> vnc://hostname.local

as a way to connect. You can have the save the vnc://hostname.local in the connect to server dialog box to easily use in the future.

You can also start a screen sharing session using Safair. Enter the following URL in Safari

vnc://hostname.local

and Safari will ask if you want to open screen sharing. You can create a Safari bookmark with the vnc://hostname.local address

NOTE: If you are using a VPN client on either Mac, it is possible this will not work, depending on whether the VPN client bypasses the VPN server for local LAN traffic. Many Corporate VPN servers do not allow accessing the local LAN when VPN'ed into work (my situation with the company I work for).

If you are using a utility such as LittleSnitch, it might block your Screen Sharing connection.

If you are running an Anti-Virus package, the firewall provided by the A/V package may block Screen Sharing access. Also it is ill-advised to be running a 3rd party anti-virus package. Apple provides rather good malware protection, especially when you use some common sense about what you install on your Mac, and the websites you typically visit.

One more thing. The iMac and the Macbook want to be on the same LAN. For the vast majority of users this is generally true when both system are in their home, however, sometimes it is not.

If you have 2 routers in your home, you could have 2 subnets (that is to say 2 Local Area Networks). Say the broadband provider gives you a WiFi router, but you decide to run Ethernet to a 2nd WiFi router to give your home better WiFi coverage. If one Mac is connected to the broadband's WiFi router, and the other Mac is connected to the 2nd WiFi router, they would be on separate subnets, and not able to see or talk to each other because most simple in-home discovery protocols do not cross subnet boundaries.

There are ways around this situation, such as putting the 2nd WiFi router into "Bridge Mode", which is either an actual router setting, or you turn off the router's DHCP server, and/or the router's NAT server. Again this would ONLY be for the 2nd WiFi router that that we are imaging is connected to the broadband provider's WiFi router.