Come up with a plan. The plan should include the order in which you want to do things (based on skills, parts availability, cash flow, what the car needs now, what can be overlapped with other work, where professional services will be required, where you need to learn something first...). It will make amusing reading when you are finished.
Beyond that, in no particular order...
A lot of what goes into the plan depends on what you want to end up with in terms of a completed project. If you are looking to show the car at concours events, go slow with disassembly and any paint or rust stripping. There are lots of paint daubs, date codes, part numbers, labels, tags, markings, colors and finishes that you will want to document as the car comes apart. I would recommend joining the Mustang Club of America for at least a year to gain access to their judging and finishing guides. They are invaluable for a detailed restoration. If you aren't worried about originality, you don't have to worry about all of that, but it is nice to at least record what you find for others.
Photos - as others have stated, you can't have too many. Spend some time looking at others photos as well. These cars weren't built exactly the same when they were new, they certainly aren't the same after 40 plus years of maintenance and modifications. I like to put a color strip with known colors in the photo for areas where getting the colors right is important.
Heavy duty ziplock bags - bag and tag everything as it comes off the car. You never know when you will get around to putting the car back together (trust me - I'm 9 years into a 2 year process...). Memories fade, and god forbid you have to move before you are done.
Bins - open top, closed top, clear, opaque - whatever. Bins are awesome for collecting and storing parts as subsystems. Things don't get lost nearly as easily or as quickly if you put the little parts in ziplocks and store them with the bigger parts in bins. Bins are tidy and they make it easier and safer to move parts around your shop if you have limited space. Make sure they are solvent safe and strong enough for the task. Parts can be heavy, awkward, dirty and rusty. Look for bins you can stack to save floorspace. There are lots at McMaster Carr and Grainger.
Label maker - for your bins and bigger parts that don't fit in bins.
Tag all of the connectors of any wiring harnesses you remove from the car. Use something that will not get knocked off, become illegible if wet, and stands up to solvents. Photograph any cable ties/harness anchors and locations. Measurements from a fixed part of the body helps as well so you can sort out holes for cable anchors vs. holes for fasteners or drains.
Get to know (and trust!) a Mustang parts supplier. There are lots out there, and many will be happy to give you a bit of a discount if you work through them for parts. Be loyal! Shopping around for price is time consuming and you run the risk of losing a valuable source of knowledgeable advice. Don at OMS is highly recommended.
read Read READ everything you can. There is a lot of (mis)information out there, so checking multiple sources and sorting through differences is important if you are looking for a competitive show car. The most basic reading the the shop manuals and parts manuals for your car. Parts manuals are more valuable to me as they show exploded diagrams of parts so you have a much better idea of how things fit together - and in which order. There is little more frustrating than doing a fantastic, meticulous job of putting something back together only to have to take it all part again because some other assembly had to be done first.
Safety equipment - gloves, kneepads, blankets, non-slip flooring, respirators, foam pads, grounded power cords, fluid appropriate containers, safety glasses.
Space - there is never enough, but if you have some hard limits, go read the part about bins. You will need AT LEAST enough space to work on the car with both doors open. The more free space you have around the car, the less likely you are do damage the car moving things around. You will also need space to clean parts. I live in Minnesota, so the driveway only works for part of the year.
Tools - Buy the best you can afford. You are better off spending more for the tools you will actually use vs. getting the biggest, shiniest sets of things that have sockets or wrenches in sizes you will never ever use. Plus you have to store all of those tools you don't use. Hold the tools before you buy them. Squeeze them as hard as you can in your hand. Feel for sharp edges, raw finishes, etc. Make sure you can read the markings on the tools as well. You are going to use them a lot, may as well buy something that won't cause aggravation. As far as power tools go, I prefer electric tools to air tools for most mechanical stuff. Batteries are pretty stout these days, and 20V systems will handle most tasks with ease. You don't have to drag a hose or cord around, which makes everything easier and safer. Less to trip over, and you won't be knocking things over on the other side of the shop when you pull on the hose or cord. Also - hoses and cords are either waaay too long or not quite long enough. Gear wrenches, flare nut wrenches, flex ratchets, flex combination wrenches, crows feet wrenches are all very handy to have on hand. Also, don't be afraid to make the tool you need.
Lights - flat LED lights are indispensable for working on cars. They fit most places, won't roll away, provide plenty of light, won't burn the skin off your arm if you touch them, won't burn paint or melt stuff, are rechargeable, and won't explode in a ball of flame if gasoline gets dripped on them.
Jacks, jack stands - low profile floor jacks with rubber pads are handy for heavy lifting. Smaller, lower jacks that fit spaces your floor jack can't reach are handy too, particularly if you are putting the car on jack stands. If you lift the rear and put it on stands, you probably can't get your big floor jack under the front. Heavy jack stands with rubber pads or wood blocks are plenty to keep the car in the air and are safe for finishes.
Be kind to the environment - please don't dump solvents, oils, paint, coolant, parts, etc. into places they shouldn't be. Check with your local recycling or hazardous waste facilities to understand the laws in your area and how to dispose of problematic stuff. I batch stuff up and take them in every few months for proper disposal.
Keep some funds available for that must-have, rare as heck part that comes up once in a blue moon and be ready to pull the trigger when that part crosses your path. You don't know when another one will come along.
A good sense of humor - stuff will go wrong. It always does. You will find things that need attention you thought were ok, which adds cost and delay. It's a hobby - the process is most of the fun. It's also ok to walk away from the project for awhile.
Hope this helps...
