I think investing in HM loans (aka trust deeds, mortgages) is one of the best most secure investments. Much more control then the stock market, much better returns then bonds, less hassle than rentals. It's like a dividend because you get a payment every month. Vs investing in say Index funds they go up and down like a roller coaster and you get no monthly dividend.
If you are going to invest through HM brokers to bring you deals, #1 you have a choose a HM lender with a lot of experience and integrity. Find out how long they have been in business, do a background check, check their license, get references, etc... Best to find one that has been though multiple real estate cycles (didn't just get in the business in 2009).
There is a lot to look at when choosing which deals to lend on that are bought to you. You need to cherry picky the best. If it's a flip you need to look at the flippers track record, get some links from Realtor.com or Redfin of before and after from flips they have completed. Check how much cash reserves they have in the bank. Make sure there is a personal guarantee. Check credit, get a loan application, get a rehab cost estimate, etc...
The #1 thing IMO is getting the valuation right so in case of default you have plenty of equity. You need to find a way to really zero in on the value, this can be trickier than it seems especially if the property is out of your area.
Also you need to get familiar with everything you need in the loan package (note, deed of trust, title insurance, hazard insurance, business purpose statement of loan, HUD 1 settlement statement, etc...).
Unfortunately there are very few books or seminars that address passive investing in mortgages and trust deeds. The best one I have seen is George Coats "Smart Trust Deed Investment in California". It's a 22 year old book though. The main concepts apply in all states. It is very difficult to get properly educated on all the risks related to trust deed/mortgage investment. There are a lot of landmines to watch out for. Paying a experienced long time mortgage attorney in the state you plan to lend in to sit down with you and go though everything that can go wrong may be a good step. Papersource has some decent info on not investing that can be applied to DD in HML.
I personally only invest in 1sts, only investment property (NO owner occupied), only SFR's, and only in a house I wouldn't mind owning if I have to take it back.
I had invested in real estate for a decade+ before I started investing in trust deeds and I also have a background in conventional mortgage lending. And I still find it overwhelming sometimes with everything you have to look at to mitigate your risks before investing in a trust deed/mortgage. And I still am probably taking many risks unbeknownst to me ("the unknown unknowns"). If you are totally new to real estate it may be daunting to invest in private mortgages. Or ignorance can be bliss until something bad happens. Most people that I know that invest in notes and HML's, had invested in real estate for many years prior. So they had a foundation in real estate concepts.