What to look for?
The portal looks like this, it is 3 tiles high and impossible to miss:


How to get there?
You'll notice that after a minute the level sections start to change - there are variations from the previous sections and brand new ones. Most noticable are disc shooters and moving saws. You can see Timen standing on a disc shooter in this image:

In order to get that far you'll need to pass the minute mark, so you must collect at least one set of items. Each set gives +30 seconds(the maximum can be 1 minute at any time). Due to the random nature of the game, the stars might be misaligned and there might be no chance to collect a set in a minute, they might be aligned and you can get a set after 5 items.

You can expect to get a portal around 2 minute mark, the game is random, so you might get lucky and get one little after 1 minute.

In order to make it to the portal, you have to Git Gud!

Tips:

• The level is seperated in certain sections that are seperated by holes. The sections come in few variations and repeat randomly, after 1 minute mark more different sections come into play. Try to Land on the very edge just after the hole and learn the sections from there.
  
• Timen jumps only after he has landed for a split second, practice constantly jumping as fast as you can on the flat section on the level start.
  
• The jump is 4 tiles long if you jump on a flat surface, and 3 tiles long if you land a block higher. Keep this in mind when trying to land on the very edge of the platforms. If you are really bad at eyeballing this you can try to place the mouse cursor where Timen would land after a jump.

One you make it into the portal, you get a bonus level (yay! extra points!):