Most convenient is probably Zelle.My favorite, though, is old-timey cash in hand, used small bills that have been sweated and sinned over. 

Our intention was to find a BRRRR property that we could move into (so we only had to put 5% down) had a separate entrance to the basement (for a mother in law), and something we could put sweat equity into.

Keep in mind there was A LOT of sweat equity that went into most of these. 

I only rarely flip a house, but when I do flip, I buy an extremely cheap rundown property and add value to it through sweat equity improvements.

The partner could put up the cash on the front end if you’re able to provide some sweat equity in finishing the basement and finding the renters.

A lot of folks forget to do this, so their investing actually involves a lot of sweat equity that isn't accounted for.

I'll be able to put more "sweat equity" into my projects which I like!

With a turnkey property, that sweat equity was earned by the turnkey operator at the closing when they sold it to you.  

BRRRR was sweat equity via rehab. 

Don't sweat that, just go acquire your first rental @Mark Smith!