You can start however you like. The number of rungs per year depends on how frequently you want cash to come available (a bond to mature). Once every month? once every quarter?
Another factor is how often you want to go through the process of finding and buying.
One method is as you describe... buy 4's,8's, etc all in one sitting and then next month when the 4's mature you buy the longest rung in your ladder.
4 weeks later when the 8's are maturing you do the same.
Other's buy their longest rung now, wait a month and buy another longest rung, etc.
The difference is in how much your cash earns while waiting to buy a rung.