A spending plan is just exactly that, a plan for how you will spend all the dollars your earn and those given to you (including, hopefully, directing some to savings). To make a change in your spending plan such as adding in health insurance costs or taking on a new car payment, it helps to be as inclusive as possible when calculating all of your monthly expenses. In other words, don't write in only the premium cost, but include some for co-pays and co-insurance and deductibles required by the plan much the same as you would not include only the car payment, but also gas, maintenance, and auto insurance coverage. See the FAQ "What are some tips to make developing a spending plan easier?"
First complete a spending plan worksheet that reflects as closely as possible where your money is going now, then do another spending plan worksheet that projects how and where you want to be spending money with the new item in place going forward. You will have to cross some things off and move dollars around to allow for a new expense, but be sure to keep your spending plan realistic above all other concerns. If it doesn't work out and you will be spending more than you earn, you need to know this as soon as possible and give yourself time to make a different plan. The University of Maryland Extension provides a Spending Plan Worksheet.
A close look at the total expenses of any health insurance plan according to the way you expect to use the plan is needed to work into your spending plan new or different health care coverage costs. The University of Maryland Extension also provides a Health Insurance Comparison Worksheet.
If you find that you do not believe that you can afford adequate health insurance, don't panic! There are subsidies and lower cost or no cost programs for which you may qualify. See also the FAQ "What health insurance plans can I enroll in?" and more information at Healthcare.gov on keeping healthcare costs manageable.