Aspiration affects syllable-initial GA /p,t,k/. When GA /s/ is the first consonant of the syllable, there is no aspiration. This accounts for the presence of aspiration and approximant devoicing in the words in the first table below. Aspiration is present in the first three lines of words; there is both aspiration and approximant devoicing in the second three.
The words below have no aspiration and/or approximant devoicing. Note in particular the lack of aspiration in words like spaghetti, stability, skill, etc., and the lack of approximant devoicing in display, distract, obscure, where /s/ precedes the stop in the onset.
Ambisyllabic GA /p,t,k/ are not aspirated. While aspiration is always present in coupon, latex, acorn, it is absent in open, later, acre (except in a very formal style of pronunciation). The /p/ in open, for instance, is subject to WSP and as a result is ambisyllabic, and therefore fails to be aspirated. By contrast, the /t/ in latex, which is followed by a strong vowel, is not affected by WSP, and is aspirated. Further examples are found below. Thus, [k] in reckon is similar to AN [k] in rekken, and [p] in happy is similar to AN /p/ in Appie.
It should be noted that for most speakers, approximant devoicing (as opposed to aspiration proper) does apply to ambisyllabic /p,t,k/. In words like April, liquid, accurate, microphone, where /p,k/ are ambisyllabic by the WSP, (/(eɪ(p)rəl)/, etc.), approximant devoicing devoices /r,w,j/ and /r/, respectively.
For all speakers it is the case that /r/ is devoiced after /t/, not just in words like nitrate, inquest, microbe, where /t/ is syllable-initial (/(ˈnaɪ)(treɪt)/, etc.), but also in words like country, contra, entry, where /t/ is ambisyllabic.
Try to pronounce a brief voiceless [h] as you release the closure for /p,t,k/ in words like pay, tea, key. If this fails to produce the right results, compare the articulation of Dutch /p,t,k/ in words like pop, tot, kok. You will find that while the initial stops are unaspirated in Dutch, the final stops may be aspirated. If they are, try to pronounce the initial stop in GA pa, Tom, calm like the final Dutch stops. Avoid overgeneralizing aspiration to non-aspirating contexts like those in spin, steel, school. Remember that aspiration is strongest in accented syllables and longer after /k/ and /t/ than after /p/.