I've mixed from chemicals for years. It's easy, and much cheaper. Buy a couple of bottles of raw chemicals, and you're not only set for life, but you never have to worry about running low. You can mix your own "dektol" too, with a couple of extra chemicals--potassium bromide, which is an anti-base-fog chemical not needed for film, and Arm & Hammer washing soda from the grocery store. It wasn't necessary to buy that borax you probably bought--washing maching borax is fine.
For years I used a cooking spoon set for quantities, a method outlined in one of the 70s photo magazines, but when digital scales became cheap and easy to get, switched. For measuring the smaller quantity chemicals, take a post-it note and fold the edges up to make a tiny tray.
Temperature is not critical for mixing, but the chemicals dissolve faster in warm water. I usually use water that I'd be comfortable taking a shower in, and dissolve the chemicals in about 1/3 the final amount (1/3L, for instance), then after everything's dissolved I add cold water to get to a temp that is appropriate for processing, if I need it immediately. That usually means adding all cold water, then, having still some space short of 1L and the temp still too high, I add a couple of ice cubes to cool it down the rest of the way.
You are supposed to start with the metol/elon, which will be the most difficult to dissolve, and harder if other chems are present. Traditionally, one threw in just a little of the sodium sulfite first ("a pinch", they say) to prevent the metol from oxidizing, but that's not totally necessary. Anyway, metol in, dissolve it as fully as possible, then throw in the rest and mix well. I use a big plastic serving spoon for stirring.
Some of the sulfite may not dissolve right away and sit on the bottom. Don't worry about it--it will eventually. I never filtered, and I always have used it right away, even with some undissolved sulfite, and never had any problems. D76 is one of the best, simplest, most foolproof developers there is. You can use it straight, 1:1 as a disposable, or replenished. For years I used it 1:1, but in recent years I'm developing 4x5 and 5x7 in big tanks, so I'm replenishing. The replenisher is easy to mix, too.
The metol will eventually start to turn brown in the bottle. Don't worry about it. I'm currently using a batch that's over 25 years old that looks tea-stained, and it works fine.
When you get tired of D76, you already have the chemicals now to make D23 and split D23, both of which are incredibly simple.
Be aware that home made D76 gets a bit "hotter" (more contrast and density) after a month or so, then calms back down. If you know this, you won't be surprised and can compensate when you see it. The pre-made stuff has buffers added to prevent this.
