Basically play "human assembler", write down the source code as assembler mnemonics on paper, then use a lookup table to convert the assembler mnemonics into he

Basically play "human assembler", write down the source code as assembler mnemonics on paper, then use a lookup table to convert the assembler mnemonics into hex machine code numbers, and type those numbers into the computer, while keeping track of jump target and subroutine start adresses.