The pronunciation of "thish year" instead of "this year" is "sound change in speech flow". This kind of sound change is caused by the fast pace of speech.
When you pronounce the "s" sound, your tongue tip is near the back of your upper incisors, and when "j(the 'y' sound in 'year')", your tongue tip touches the back of your lower incisors.
Thus when you consecutively pronounce "s" and "j", there is an almost largest span of distance ever possible that your tingue tip may move through during speech, which is obviously inconvenient for continuous pronunciation.
If you speak at a slow pace, there is little trouble in pronunciation, you only make the sounds one after another. But when you speak quickly, it then becomes a stiff challenge for your tongue tip.
Therefore your tongue tip cuts short the distance it has to move through: you begin to make the "s" sound befor it reaches the accurate position where a normal "s" is made so that the changed "s" sounds somewhat like a "sh" sound.
Sound change in speech flow takes place just naturally. You do not have to do it deliberately，but you only pronounce as normal. When you speak slowly, you get the normal sounds; and when you speak quickly, the altered sounds will automatically take place.