There were plenty of assemblers around, some even native. Heck, I wrote both an 8008 and an 8080 cross-assembler (in FORTRAN, naturally). It's not rocket science. One friend of mine wrote his assembler as macros for a mainframe assembler. That, at once, gave him all of the advanced facilities of the host assembler. Wish I'd thought of that...
So, I wrote my primitive little 8080 assembler, in Fortran-77. I started using it the next day, and my manager asked where I got it. After I told him, he said that I should have asked him first, because he could have put together one using editor macros pretty easily - in less than a week, he estimated. The fact that I knew nothing about macros notwithstanding, I noticed I thereafter got a lot more autonomy when I told him I wrote mine in a single (longish) day...
~~ Mark Moulding