The key to success as always is: DO YOUR GOD DAMN RESEARCH!
Once you've done enough research, you should have a general idea how each part can be made thus having a rough estimate on time.
I often hear people running out of time and are still working on their costumes the night before, during a convention and on the way to the convention. I personally think that is cutting it way too fine. When I go to conventions, I would like to relax, have a few drinks, have a few more drinks and then wake up for the next day of the con as oppose to stressing out and finishing my costume at the last minute.
When I plan a costume, I always leave around 6 months before the convention. This includes time lazing around, motivation breaks etc. I understand sometimes that is not possible so sometimes you just have to soldier through.
Time tracking application
I've recently started using time tracking applications to track how long my build times are. I started off using an android app called "My Work Clock" but it was a bit too basic and it did not have any web interface. In the last couple weeks, I've moved onto using "TrackingTime" free version. It's not bad for what I need. It's got a web interface as well as an android app. The main thing I use this is hour tracking as well as planning. I set myself an estimate and see if I can achieve this. The more I do this, the more accruate the estimates would be. Since I am new to taking commissions, I have absolutely no idea what anything would cost and this kind of application helps me create a better picture and give a better/more accruate quote.
How do I get motivation?
I understand sometimes it's extremely demotivating to work on something for so long. You may lose steam and hope or even hit a wall. Like for example, just the other night, I created some knee pads that took me all night to design and craft. I tried it on and it was fine but when I tried it on with my muscle suit, it was wayyyyy too small. There was literally no way I could alter it to make it fit so I have to create a brand new one. It also didn't help I threw away all of my templates. I was moaning about it for a good couple hours but I soon realised the work isn't going to do itself and I had to bite the bullet and just start again.
I tend to set myself some time (at least 1 hour a night) dedicated to working on my costume. Even when I didn't feel like it, I do something that's related to the costume whether it's making another bit, re-sanding stuff or doing more research.
I think the best way is to have crafting days with some friends. You'd be surprised how much you accomplish when other people are also working around you. If that's not possible, the next best thing is to have some background noise whlie you work. My personal preference is to sit online with a few friends while they do their stuff (gaming or craft) and just chillax while I work. If you don't happen to have any friends, not to worry, just put some music on or a no brainer TV show on like South Park or something. Click on the discord photo or click here to join my discord server!
Creating your costume can be as cheap or as expensive as you want it to be. The majority of the money I spend for crafting goes into tools. Since I'm still quite new into this, there are a lot of tools I want/need. Before you make any rushed decisions, just have a think; do you really need it? How much will you use it once you have it?
For example, I've been thinking about purchasing a bandsaw as well as a vertical drill for a while now but I couldn't really justify it. The saw would save me a lot of hassle messing around with a jig saw but it wouldn't save me enough time to make it worth it as I normally just work with foam that's thin enough to be cut by a normal box cutter.
The belt sander was on the opposite side of the scale. Initially my thoughts were, I already own a Dremel, why would I need a belt sander.... until I used one. Belt sander is love, belt sander is life. In fact, I love it so much, when I was choosing a place to move to, it had to have an outdoor area where I can set up my very own belt sanding. Yes, it is an expensive bit of kit but I sure had my money's worth of use out of it.
Planning your build
Make sure you list out everything you plan to buy and keep track of what's been bought so you don't duplicate your purchase. This may seem too obvious to state but I've bought duplicates before because I was in a rush for the item. Once you have a rough list, you can buy things in bulk. For example you could probabaly estimate how much worbla or foam you need for your build. This could save you £££ on postage. You don't need all your materials right away to start your costume. This may seem contradictory to the previous point but it's not. For example, you don't really need to buy foam and worbla at the same time since you nearly always have to finish your foam before you start the worbla it. Since you plan your costumes well in advance, you could wait until the next payday before ordering your worbla.
When starting a costume, I usually list out all the stuff that I need to buy from ebay and see roughly how long each one would take to arrive. The cheaper it is, the longer it's going to take to arrive because it's probably made by one of my cousins back home... in China.
Compare the market
Don't just limit yourself to your usual supplier. Yes, you may be that loyal guy who loves supporting your favourite shop but at the end of the day, I'd rather spend the money I spent on another drink at a convention. For example, until recenlty I've limited myself to the usual UK supplier Coscraft for worbla. After being introduced to CosplayMats, I found out the worbla is £3.50 chepaer than Coscraft. There is however a small catch. POSTAGE! When ordering 1 or two rolls, the difference is minial but if I was to order 4 sheets (and stock up for next time), I save enough to buy another roll.
I've recenlty created a spreadsheet where I kept track of everything I had to prucahse for the costume including materials, tools and whatever that's gone wrong. This is extremely useful for future reference because now I have a more accruate estimate on how much a cosplay would cost and it's especially good for commission estimates.
TL;DR Spend time reseraching, plan your material purchases, stop being a bitch complaining how hard it is and JUST DO IT!