The following are major contributors to a map's difficulty.
The more notes per second you need to hit, the more difficult it will be for players to keep up. Newer players need time to see a note, then coordinate their motion to reach it. They will not focus as much on upcoming notes until the current one is hit. Experienced players can more easily read several notes in advance and plan out their motions.
Perhaps more important than note density is the speed and complexity of motions. Newer players will want to ease into the game with slow and smooth motions that don't quickly redirect very often. Experienced players will want a more brisk pace of movement with bursts of high speed excitement.
On easier difficulty levels, it should take more time to reach the outside of the comfort grid, and faster movements should stay closer to the middle of the play space. On harder difficulty levels, the outer edges of the comfort guide are great places for impactful motion.
Non-symmetrical two handed patterns can be tricky for newer players to handle, as are crossed over two handed notes. Use them sparingly if at all at lower difficulties — ensure there is some space before such a pattern so that the player has time to read the pattern and plan their motion. As the difficulty level goes up, you can add longer and more varied two handed note patterns.
Walls are great for adding full body movement to a map, but newer players may have trouble reacting to a wall while hitting notes. Easier difficulties should have fewer walls and either simple note patterns or no notes at all around them. Harder difficulties can ramp up the complexity of notes around walls, as well as increase the frequency of walls.
Most mappers find it easiest to create a map at the highest intended difficulty, then copy it to a lower difficulty and make edits to decrease the difficulty. The following guidelines will give you a rough idea of how hard each difficulty should be.
This is the limit of how hard a map can get. There are no specific guidelines for this, other than don't push it too far by making the player do unhealthy or dangerous things (such as repetitive jerky motions, whether that be with notes, rails, or walls).
Similar to Master, but reach is brought in a little bit, and the most complex patterns are simplified. This can be achieved by slowing the motions and/or simplifying some two handed patterns (either by removing a hand, making it more symmetrical, or uncrossing some crossed over notes).
This should be much easier than Expert. Intense sections should have noticeably slower motions and decreased note density. It should take some time to reach the outside of the comfort grid at this difficulty. Two handed patterns should be simple, and you shouldn't swap between hands too often in a short time.
This difficulty should play like a gentle groove. Note density should be quite low, so consider removing the notes between downbeats. Intense motions should be replaced with gentle ones that take time to reach the outer edges of the comfort grid. Avoid tricky two handed patterns. Don't make the player suddenly reach to the opposite side of their body — instead, start on the natural side of the hand and direct the pattern over. Sudden crossovers are difficult for inexperienced players to react to. Try to remove crossed over two handed notes unless the section is slow enough that the player will have time to react to them.
At this difficulty, players need to focus when dodging walls, so consider removing notes that are around them. Wall density should be low enough that the player has time to comfortably pivot and move.
Anybody (including players who are new to video games and VR) should be able to pick up an Easy map and get through it without failing. Note density should be low — stick to downbeats, and on higher tempo songs, remove notes on less accented beats to give the player time to read upcoming notes. Motions should be slow, and give players plenty of time to lean towards the edge of the comfort grid as the patterns move there. Patterns should be simple and mostly one-handed, although some symmetrical two handed patterns can work if there is space beforehand to read them. Do not use crossed over two handed notes. As with Normal difficulty, do not make players suddenly reach to the opposite side of their body. It's best to start patterns off on the natural side of the hand and slowly move around the play space.
If you are an experienced player making an Easy map, you should probably be bored while playing it. Just make sure you have a variety of motions, even though they will be slow.