How do you write 'to', 'then', or 'via' in IFR clearances shorthand symbols?
Do what works for you, and omit what you can. For example, "N123 is cleared to KABC" can be condensed to "ABC"... assuming the clearance is for you, why copy your own tail #? You're copying a clearance, so "is cleared to" is, to me, entirely implied by the fact that there is something written. 

"Climb &amp; maintain 7,000" is for me an "M" with a horizontal line through it, and "7". If it was 5,500 I'd either write it as 55 or 5.5, since neither 550' nor 55,000' would make any sense. "Expect FL210 in 10 minutes" is "x 210 / 10" -- or "x21/10" -- because I know that I won't be cruising at 2,100', "21" isn't ambiguous.

"As filed" is AF, "climb via the (filed) SID is "via". Direct is a "D" with a horizontal line through it. (Or an arrow, but omitting the arrowhead saves writing &amp; is equally clear that this is a symbol with specific meaning, rather than a letter.) Some people use "DCT" for direct, which is fine, although the D symbol will never be confused for being a navid I need to locate. "Right" and "Left" are circled R and L, if needed.

"Squawk 1234" is "sq 1234" and "departure on 126.55" is copied as just the frequency, unless there are several - then "dep 126.55 mtr 121.65 5 pr" would record the need to also call Metering 5 prior to start. If it's Ground or Clearance I need to call 5 prior, I'd probably just abbreviate those as G or CD, without the frequency unless there are more than one -- I have all of those on my EFB so "Gnd 121.9" is no more info for me than just "G".

For me, the recorded clearance isn't a document I need to refer back to months from now so I can recite verbatim what the controller read to me; it's a memory aid so that for the next hour or two I can know the pertinent elements without having to remember them. So numbers like squawks, frequencies, headings, and altitudes are vital. Fixes and procedure names likewise, which is easier with a printed copy of the filed route handy, where I can underline those elements I was given.

For example, if the filed route for KABC to KXYZ is to the ABC VOR, then to AFFIX then to BBFIX then V123 to DEF then the WHOZT1 arrival, the clearance might be "on departure fly heading 150, radar vectors to BBFIX then as filed," so I'd write "H 150 RV" then underline BBFIX, and below that I'd write "AF". After I'm done reading that back, I'd line out the ABC VOR and AFFIX (that weren't part of the issued clearance).

Your instructor may have a different way that's preferred, and in that case go with what you're being taught. But over time, this is what I've found works well for me. Less writing, no need to scramble, what I need &amp; no excess when I come back to look at it later.