(for more information, take a look at the User Guide)

1. declare a Specification object

import org.specs._

object helloWorld extends Specification

2. a Specification is just a list of examples:

import org.specs._

object helloWorld extends Specification {
  "'hello world' has 11 characters" in {}
  "'hello world' matches 'h.* w.*'" in {}
}

3. back-up your examples with expectations:

import org.specs._

object helloWorld extends Specification {
  "'hello world' has 11 characters" in {
     "hello world".size must_== 11
  }
  "'hello world' matches 'h.* w.*'" in {
     "hello world" must be matching("h.* w.*")
  }
}

4. compile and run the class named helloWorld: scala -cp specs-<version>.jar helloWorld

Specification "helloWorld"
  specifies 
  + 'hello world' has 11 characters
  + 'hello world' matches 'h.* w.*'

Total for specification "helloWorld":
Finished in 0 second, 63 ms
2 examples, 2 assertions, 0 failure, 0 error

5. now, you can more structure to your specification, by grouping your examples:

import org.specs._

object helloWorld extends Specification {
  "hello world" should {
    "have 11 characters" in {
      "hello world".size must_== 11
    }
    "match 'h.* w.*'" in {
      "hello world" must be matching("h.* w.*")
    }
  }
  "Good bye cruel world" should {...}
}

6. You can also use a JUnit runner to run your specification inside Eclipse for example (using the JUnit 4.5 library). Add:

import org.specs.runner._
class helloWorldTest extends JUnit4(helloWorld)

// OR declare your specification as a class and mix-in the JUnit trait
// in that case, the specification can still be run on the command line with
// `scala -cp specs-<version>.jar run helloWorld` 
class helloWorld extends Specification with JUnit { ... }

Note: helloWorldTest is declared as a class and not an object to be instantiated properly by JUnit runners.