how do you convert binary to a form in which you can perform subtraction?

You can use a common method involving 1's compliment, in which the binary number is inverted and then added, which overall gives subtraction. E.g. 8+3 =>> 8₁₀ = 1000₂ and 3₁₀ = 0011₂. 1's compliment would give -3₁₀ = 1100₂, as this inverts 3. this means that the 0s change to a 1 and the 1s change to a 3. you can now either add these binary numbers together and add 1 at the end, or use 2's compliment instead of 1's compliment, where a 1 is added to the inverted 3 now. so -3₁₀ becomes 1101₂. addition gives 1000
                                                                  1101
                                                                10101 the underlined 1 is discarded by the machine as the stored value is only a 4 bit value, which means the answer here is 5.