http://www.elmergates.com/egamu/egamu_link_page.htm
I think his three tips would be:
(1) quiescence (2) periodicity (3) dirigation
Which in modern language means (1) remove or zone out all distractions, (2) work at the same time each day, (3) pay deliberate attention to each aspect of the problem in turn in your imagination. (4) Repeat.
Step (3) helps to build the relevant mental structures. Visiting them habitually will strengthen them.
'Each aspect' will include mental images (visual, aural, kinaesthetic), concepts, ideas (including all the relevant constraints and conditions that must be met). Generating mental images is important as the mind works best with things that are translated into sensory terms.
Eventually a conjectured solution will pop into your conscious mind, possibly at a time outside the working hour. You may know it is coming before it arrives.
He might also add that thinking draws from one's personal store of vitality. So it may be necessary to adjust sleep, diet, exercise and any addictions you have in order to conserve this.
Being unstressed is vital. Partly this means only working on important problems and working with the intention to do good. It also means switching to a different problem if you get bored of the first one. (You can go back to it another day).