Now that I have finished my piece, I thought I'd share some of the things (and tips!) I discovered/learnt on the way, both from experience and from EC in the hope that it might help some others in my situation, starting out with no prior knowledge. A few things aren't readily apparent in the tutorials and I think you only find out from having a go... but obviously mistakes happen along the way! They also may be obvious to some, but hopefully can stop others making the same mistakes I did.

Before starting it should be reinforced that although the process is quite straightforward in terms of the steps undertaken, it is extremely unforgiving so expect to make mistakes. With that in mind certainly make sure you have the time available and have enough resources for buying more materials on top of the initial outlay for equipment (I'm not sure doing it on the cheap is super viable). I did it in a pretty confined space, so whilst not essential having a good work area is beneficial.

Additionally the EC technical help team are excellent, and always seem happy to help and chat through and issues no matter how big or small. They are a great one stop shop for all things composite and I sourced all my equipment and consumables from them, mostly because they appear to stock everything one needs in one place at a keen price, but it also seems a fair bargain if they are providing free technical assistance.

I'm no expert so I'm sure people can correct me if I am misinformed. Also I've covered all stages of the process I took, but am posting in Resin Infusion so it's all in the same place.

1. The Pattern Part

• Firstly, and I can't stress this enough, get you plug/pattern/original part as good as you can make it. The original piece I was using was in pretty poor shape (rusty, nasty welds etc) and whilst I cleaned it up I made the mistake of thinking it would be easier to sort out the mould after. It was not. Whilst it is quite easy to sand the Uni-mould gel coat it was laborious and extremely time consuming, and took months of sorting before the mould was ready to use. After I ruined the first mould (more on that later) second time around I got the piece  sandblasted back to bare-metal and filled in all the holes, smoothed out welds, etc.
• Be careful using paints you use to finish the part. The polyester/vinyl ester resins contain styrene solvents which attacks paints. I was a bit unlucky with this in that my first choice primer/paint/clearcoat (1k) combo seems like it would have been unaffected but I ran out of the primer mid way through (I was using a highbuild primer to fill in minor defects) and instead of waiting to order some more used another primer I had to hand. The styrene attacked this second primer, lifting the paint onto the mould surface and causing wrinkles in the gelcoat. This took a few weeks of removing the paint from the mould (gave a very good release though!) and sorting the wrinkles. Areas without the second primer were unaffected. However, I wouldn't risk using paints (or at least test them first), and Easy Composites do epoxy coatings for finishing patterns which would be better to use. 
• If using the filleting wax for filling gaps on the part surface be aware that it can give quite a rough finish, especially if just smoothing out with your fingers. I did this on the first attempt and it took quite a lot of time sanding flat and filling in after I completed the mould. Second time I used body filler so I could get a very smooth finish  - although I understand these can also be affected by the styrene solvents in the resins. Mine were fine (I used P38/BigBoy) but worth consulting with EC.
• Likewise when using flashtape to cover holes, it is worth cutting it neatly around the hole, as per the EC tutorial videos. I did it fairly roughly on the first mould, thinking I could just easily sand back but again this is just making more work for yourself later on. Note that every surface imperfection on the part will show on the mould. 

Its just a rule of thumb to give people some guidance on where to start with cosmetic panels, not everything needs proper engineering. If you're making a new set of fenders or some other direct replacement panel for your car, which is what most people here are doing, matching the thickness of the metal is going to give you a pretty good starting point. From there you need experience and understanding to know how certain layup are going to perform, and youre only going to get that by laying up some panels. So having some hint of where to start isn't a bad thing.

Yes, it is technically not correct. And it is a bad way to build composites if you are going beyond cosmetic panels, and dangerous if you are getting into critical structural panels without any other knowledge or experience of how that panel is going to perform. But it works just fine for what most people here are doing, and considering the topic title is 'Notes from a Beginner' - I dont think it needs to be removed.

How about replacing a fiberglass part with carbon? Trying to figure out how many layers I need for my hood.