Before BGP advertises a route it must be able to find it in the routing table.
This presents a problem when it comes to advertising a summary route as the summary you wish advertise will not be acutally listed in the routing table. Your subnets will be but the summary route wouldn't be.
To get round this you first configure a static route for your summary route which in turn is added to the routing table:
R1(config)#ip route 192.168.0.0 255.255.0.0 NULL0
This adds a static route pointing to NULL0 in to your routing table. Remember you will have more specific routes to the subnets you are wanting to summarise, such as 192.168.10.0/24, 192.168.20.0/24 and so on. As the routing look up works on 'Longest Match' this NULL0 static route is ok as you will/should always match the subnet with in the summary static route you applied.
With the summary route now installed in the routing table you can go ahead and advertise the summary in to BGP:
R1(config)#router bgp 500
R1(config-router)#network 192.168.0.0 mask 255.255.0.0
Here, you are advertising the class C /24 subnets as a /16 summary route. As there is a route in the routing table for this entry BGP will advertise it.