This can be done two ways:
1. **Easy way:** Purchase a new 700c wheel with coaster brake hub from your Local Bike Shop or the internet. It may be hard to find this as it is not a particularly popular option, but it's not uncommon to find one at a well-appointed bike shop.
2. **Hard way** : Purchase a new coaster brake hub, and rebuild your existing rear wheel (or have it rebuilt--wheel building is a complex process) with the new hub. One advantage to rebuilding your existing wheel is that it will be easier to find a new or used, unbuilt coaster-brake hub. Additionally, you'll be sure to have matching front & rear rims.
Other than that, there are no other steps or considerations you'll need to take.