As the previous answers stick to the correct temp/time. E6 process has been tried and tested for perhaps close on 40 years so they will have got it right by now. Ideally the temp should not waver more than +/- .5 of a degree so it is not the easiest to deal with. Actually the 1st developer is the critical one, the rinses between each bath and indeed the 2nd developer and beach/fix stages is not as critical temperature wise, as the 1st developer. +/- 1-2 degrees will not make a great deal of difference. As well as this extra lattitude with temperature, you can develop and bleach fix for longer than the stated time by a minute or so especially if the temp drops below the ideal 100F/38c.
Whenever I use slide film which is very rare now, I would use my trusty JOBO rotary processor, but when I started out, I used a water bath in which I was able to maintain the temp by floating the tank in a water bath perhaps 1.5 degrees above the developing temp with the different chemicals alongside. If the waterbath temp dropped, I added a small amount of hot water at a time to bring it back up.
Probably the essential part before starting to process is to ensure the tank itself with the film inside is also brought up to the recommended temp. Pour developer into a cold tank can see the 100F drop to 98 or even loer and it will never catch up and the slides will be too dense.
It takes a lot of practise to get get it right every time, but you could always do training runs using an empty film tank and plain water at the recommended temp for the times suggested to see if you can get it close enough so it makes little or no difference. Take encouragement that even professional labs get it wrong, even the hallowed Kodachrome made a mess of it occasionally