Choose a vehicle that you can live with. Ugly cars are tough to complete.
Find one that has ALL the options you want and the right driveline eg. automatic vs. manual transmission. If a car came with larger and smaller engine options, then the larger engine may also have a substantially more beefy transmission and final drive, along with upgraded springs and brakes.
Find the best example you can, within your budget constraints. Keep in mind that a cheap high-mileage car may require expensive brake, steering, and suspension work, as well as possible bodywork. Anything you buy as part of the car is half as expensive as something you need to add later.
Get the shop (repair) manual for that car. You will be doing extensive work on all parts of the car and you need to know how to take it apart and put it back together. The book might cost $100 if you need to get a dealer-only volume, but that is a small cost compared to the rest of the conversion, let alone the money it will save you if it prevents one mistake.
Remove old ICE components (ex. Engine, Gas tank, Exhaust, muffler, Fuel lines and filter.) See also the Weight Reduction
It's often easier (and sometimes mandatory) to remove the engine and transmission as a unit. Consult your manual as to the recommended procedure. Having the transmission out of the car makes it easier to do motor-adapter-transmission assembly and testing too.
Many regulatory agencies require that ALL fuel lines, breathers, filters, etc. be removed prior to allowing the car to be licensed as an electric vehicle. Make sure you don't leave anything behind.
Don't forget to drain tanks, oil pans, radiators, etc. before starting to remove hoses. A couple of gallons of fluid makes a big mess, and can be very dangerous if it's flammable (fuel) or toxic (antifreeze).
Get the right tools to do heavy work. A lot of the stuff you're removing will have to go back in the car, and you will need jacks, jack stands, a hoist, and various car-specific tools to put the motor and transmission back in the car anyway. Not cutting corners here will save you time, break less car parts, and maybe save your life. Never work under the car unless it's properly supported and nothing heavy can fall on you.
Dispose of the unneeded components - but make sure you have everything you need. It might be worthwhile to keep the engine and accessories until you have the motor and transmission back in the car and spinning the drive wheels...
Remove interior components as required to facilitate batteries, charger, and other accessories. Store those out of the way.
Prepare the car for conversion.
Make sure everything works... easier to do that before you strip it all down, than afterwards. Make a list of stuff that needs repair, and get those parts.
Depending on the condition of the car: Remove all interior and trunk components that you are going to remove or replace anyway: carpeting, sound deadening, etc. Then take it to the coin car wash and wash everything that is not going to rot from getting wet... clean the engine compartment, engine, transmission, and all parts of the suspension, brakes, and chassis that you can reach. Include the trunk interior if you can manage it; any dirt you remove here is not going to fall on you later.
If the car isn't mobile then you might need to do the above step at home using a pressure washer. Prepare for a LOT of gunk to come off the car.
To summarize this is a better cleaning job than even the best detailing car wash. You have to clean the car thoroughly interior and exterior. No dirt can remain after the cleaning.
You may want to get your target battery system last, since the cost is still going down and the performance is going up. Many controllers will let you do driveway tests using a couple of 12V batteries as a traction pack.
Assemble motor and transmission.
Ensure that the transmission properly aligns with the motor and that everything spins freely.
Do a bench test using 12V battery to ensure things are working.
Fabricate motor and controller mounts as required. Re-using OEM style rubber mounts will make the installation quieter than hard-mounting the motor.
Install motor and transmission, verify that drive wheels spin... the right way.
Traction pack installation
Determine the battery placement
Make battery boxes
Cut out chassis as required, reinforce edges and install battery box mounts - make sure no brake lines or OEM electrical is cut.
Make charger mounts
Mount battery and BMS/charger components. Figure out cable runs
Reinstall the traction system. Where possible, test incrementally, so that it's easier to localize problems.
Install remaining interior and finish components...
This is still a pretty cursory list - each of these steps can require quite a bit of work.