Interest in RE is the rate/360 x the number of days the principal accrues to any scheduled payment, then the is an application to principal, the that balance x rate/360 x the days accrued.

Here's a link to the actual guidelines dated this year, just click on the part referring to conversion of principal residence.  

.† Final value defined as = [Down Payment] + [total cashflow] + [principal paydown]*The ROR formula breaks down when you have negative cashflow since you can't pay the negative delta along the way with the equity you only realize after selling (or refinancing).

I have other deals where a pay principal only payments over 84 months. 

40 years of real estate probably closing in on 300 million or more in transactions that I have been a principal in  thousands of loans that I have made with my own capital. owning 400 + rentals...

, how do many obtain the principal to buy their 1st investment property after buying the property they reside in?

Could be used as a principal residence or a vacation rental.

When you do your property analyses there are 6 major items to include:  principal, interest, taxes, insurance, maintenance, and vacancy.  

Save that money so you can buy another investment property and get more of that Cash on Cash return in the 30 to 40 to 50% (if you add rental profits, appreciation, and principal paydown). 

For example, if it's $200/month, you are looking at $2400 negative cash flow a year, so it will take 16.5 years to loose $40,000, while you will be paying a lot of principal down and you'll get some appreciation.