You have independence when it comes to designing your workouts. The only expectation is that they are welcoming, fun, and effective. Below is some information to help you in the process of planning.
If they're swimming, they're getting fit, so workouts can be presented in a random fashion and still be effective. In fact, random training stimuli, as long as the load is appropriate, has been often shown to be very effective.
That said, breaking up the year into focused periods can make planning easier and help to progress the swimmers towards a goal. Here is a simple model that can be used:
Fall: The first month should be planned on the assumption that the athletes have not been swimming and do not have a good strength or endurance base. Total volume and intensity should take this into consideration. From there an initial focus on technical development of the stroke (freestyle only or all four strokes) will not only create a technical foundation but will also mean strength and endurance is slowly developed due to the lower intensity of drills. A technical focus is certainly not mandatory but improving stroke mechanics early, before the higher volume swimming to come, is a logical approach. If focusing on technique, shorter intervals are recommended. This makes it easier for the athletes to sustain proper technique without physical and mental fatigue causing regression.
Winter: As you come out of the Christmas break, remember to back off volume and intensity for a couple workouts since some fitness will have been lost. The next 10-12 weeks is a great time to slowly build volume into the workouts through a slow decrease in drilling and increase in duration of the main set. Longer intervals are an excellent strategy here as they develop the endurance and aerobic capacity that is key for speed development.
Spring: With the technical, endurance and strength foundations built, this is a great time to up the intensity and start to do more high intensity swimming. Such threshold and supra-threshold intervals will give a big boost to the VO2max and strength which will have a positive affect on the ability to sustain speed during longer intervals. Spring is a time where you could also have some fun with open water skills and simulations.
The majority of coaches use a volume-based approach to planning the individual workouts. First, the order of workout elements would be determined. A typical order would be: warm up, kick set, drill set, main set though, this may vary with the time of year. You would determine what the plan for each of those elements is. Next would be determining the volume of the elements for each pace lane (ie how many lengths/metres) along with the rest interval. This comes with experience but remember that you can always adapt during the workout on the fly, tweaking the volume if one lane is far behind the others. Understanding the pace range of each lane from the start of the season is an important tool in planning. Some coaches will print off a workout sheet for each lane while others may use white board if available.
Another approach that can be used is to use timed segments. The order of segments would be the same but instead of a designated volume, each segment would be given an overall duration (ie 10 minutes). Each lane then continues until the coach stops them at the end of the segment time. In stopping swimmers it is common to get the first few at the wall to do a half length so all swimmers finish roughly at the same time. This approach does not require sheets for each lane but, instead, the coach addresses the full group between each segment explaining what's next.
Click here for a page of sample workouts.