When you develop slides in general, and when you don't use a rotary processor in particular, you should make some tests to see that everything works well.
There are MANY mistakes you can make especially if you are new to DIY developing in general. If you already practice B&W developing then you are already half way to success, but I suggest you try first with some test roll before you develop your holiday rolls. I also suggest you develop your rolls one at a time so that you can adjust developing times if necessary.
If all your holiday rolls will be Velvia, then your test rolls should be Velvia as well.
The key word in this matter is consistency. Once you find a procedure that works, keep that procedure exactly the same for subsequent rolls. Exactly same bath temperature, exactly same chemistry quantity and dilution, exactly same agitation pattern.
You also have to figure a way to keep baths, tank etc. all at around 38 °C.
Temperature is important ONLY during the first bath. All the other baths are "to completion", if the temperature goes a bit below 38 °C don't worry just soup the film more.
- A basin with hot water tap and a "too full" water sink. You check temperature and from time to time open the hot water tab;
- A fish tank thermometer. Those don't arrive to 38 °C but I "modified" mine to arrive there (force the temperature knob);
- An electric dish warmer: electric plastic things that have a surface which can be kept at a mild temperature. You put a big plastic container over the dish warmer.
It's important that you wait for the entire "thermic system" to have stabilized. I wait for at least half an hour. All elements (sink, flasks, tank and liquids) must go to temperature.
PE suggests two 30" water rinses before developing. They will bring the film to temperature. Don't worry about the coloured water coming out.
PS To answer your question: it is NOT difficult at all once you found your procedure. The work is all in the establishing of a proper procedure. Then it is very easy, just longer than with B&W.