It's very convoluted and you're better off reading the wiki example if you want something shown with real numbers but I will attempt to summarize.
1. Determine your overall flat damage, both physical and chaos.
a. Flat physical, flat chaos and %-based physical on the weapon will be local, as in their effect is already calculated and will be shown in the weapons description.
b. %-based increases to chaos damage on any weapons are global meaning it will apply to all chaos damage, this is used later on in step 3, not used here.
c. Flat added damage from rings or abyss jewels is also taken into account.
d. Cobra lash has an "added damage effectiveness" of 125% at level 20 which multiplies your flat damage in this step by 125%. This is special and entirely separate from normal %-based increases or multipliers from passives/gear/supports used in steps 3 and 4.
2. Convert your physical damage to chaos.
a. Cobra lash has a built-in 50% conversion rate.
b. If you had any sources of "adds X% of physical damage as extra chaos damage", this would apply here as well but in a slightly different way than converted damage.
3. Take into account all of your %-based increases
a. The unconverted portion of your physical damage will be affected by increases to physical damage or generic damage increases such as one-handed/attack/proj damage.
b. The converted 50% to chaos of your physical damage will be affected by your physical damage increases, your chaos damage increases and also any generic damage increases.
c. Any Chaos damage you have that was not converted (flat damage from your weapon/rings/cobra lash itself) will be affected by chaos damage increases and generic increases.
4. Apply all more/less multipliers
a. Calculate your total generic more/less multipliers first
b. Calculate all physical specific multipliers and chaos specific multipliers
c. The same as step 3, unconverted physical receives both generic and physical-specific multipliers, physical that is converted to chaos gains all 3 multipliers, and chaos that wasn't converted from physical gains generic and chaos specific multipliers.