How do I repair cracks in a plaster ceiling?
If the cracks have appeared recently and are still growing (in length and number) then this potentially indicates something serious.

Before you attempt any repair - which is perfectly possible using the method you outline - get someone in to have a look so see what might be causing them.

Have you put anything up in the crawlspace recently, or taken anything out?

Have you had an unusually wet or dry spell? This will change the ground conditions possibly causing the house to move which in turn could cause the cracking.