The simplest was is to just divide your total dividends for the year by your starting capital for that year. For example, if you start with $100000 in capital and earn $5000 in dividends during the year your dividend yield for that year would be 5%. Even if you haven't fully invested your total capital during the year it is still available to be invested and the total should be used for the calculation.
Once the year has ended you will have a new balance for you capital, which is the value you should use in the following year.
An alternative to this calculation is to use your average capital for the year, add your starting and ending capital values and divide by 2. You may want to use this method if you trade often during the year instead of buying all your investments only at the start of the year.
Also, another thing to consider is whether you include the dividends as additions to your capital or if you withdrawal dividends when they are paid for your own cashflow. If you reinvest your dividends, then they should be included as part of your capital.