At level 24 the primary change going from a Wolf Spider to a Crab Spider is that you want to swap your Wolf Spider attacks for Crap Spider attacks. So the simple advice would be to look at your current Wolf Spider attack powers and replace them with the equivalent Crab powers.
Doing it from scratch at level 24 you have 14 powers so I would take the following):
1: Channelgun
1: Crab Spider Armor Upgrade
2: Long Fang
4: Combat Training: Defensive
6: Travel Power pre-requisite
8: Supression
10: Tactical Training: Maneuvers
12: Venom Grenade (take the one from the Crab Spider set, not the Wolf Version)
14: Travel Power
16: Tactical Training: Assault
18: ???
20: Tactical Training: Leadership
22: Mental Training
24: Fortification
The level 18 power is a bit of a wild card. From the Crab Spider powers you can take either Arm Lash or Frag Grenade if you want more AoE (I prefer Frag but if you hate knockback you might prefer Lash) or Aim if you like it. It's also good point to take a power form the Leadership pool if that's your thing or alternatively dip into the Wolf Spider attacks for Heavy Burst if you want more AoE and don't mind the redraw.
Just dinged 24 with my soldier and want to make him a crab. Problem is, I don't know where to begin. Does anyone have a good leveling build, or at least a level 24 starter build I could use?