Since you are converting money from an existing Traditional IRA to an existing Roth IRA, you are not doing a 'back door' Roth IRA contribution. You are simply converting money from a Traditional IRA to a Roth IRA.
Looking at the steps for the back door Roth, you will only be going through the steps for part 2 -- entering the Form 1099-R reporting the distribution from your Traditional IRA. You do not need to go through the part where you enter the contribution to the Traditional IRA since the money was already there in your case.
So, to answer your questions:
1 - Do not complete step 1; you only need to complete step 2.
2 - You can enter '0' because you did not contribute to the Traditional IRA for 2020, but you do not need to complete this section at all.
3 - You are asked to provide the value of all of your Traditional IRA accounts so that the taxable portion of the Traditional IRA distribution that was converted to the Roth IRA can be calculated. This is especially important if you had both deductible and non-deductible contributions to the Traditional IRA.
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