Sorry, I didn't see you replied.
Hook up the voltmeter to the battery, red (positive) lead to the positive battery post and black (negative) lead to the negative post, make sure it reads at least12 volts, then turn the key to start and read the meter while the key is on start. It should remain over 9.6 volts. Then to check the connections between the battery positive post and the starter move the voltmeter to the very next connection and turn the key to start again and read the voltmeter. It should e within 0.2 volts of what the reading was at the battery with the key on start. Of course you will be needing to wait until this problem of the starter not cranking over in order to test and find the problem. Then move the positive test lead to the very next connection and check again. Each time you move to the very next connection, keep in mind it means the immediate next connection, so you might have 3 tests before even leaving the battery and that doesn't include the tests on the negative battery cable. For instance, the first test should be on the actual positive battery post itself, then the second test on the positive battery post cable end, then the third test touching the copper strands of cable at the positive battery post, if they are visible. Moving on, say you get to a connection. You would first test the copper wire strands, if they are showing, then the wire or cable end, and then the nut or bolt that holds it tight, then move onto testing the copper wire strands on the cable or wire coming off of that connection. Until you get down to the starter itself and test there on the bolt itself. Each connection should not be more than a 0.2 volt difference then what the voltmeter read across the battery when the key was on start. So, at an average of let's say 5 or 6 possible connections being in that positive battery cable, no more than 0.2 volts loss on each would total 1 to 1.2 volts total loss and I believe the spec is probably lower and closer to 0.1 volts. So, at the starter you should have not less than .6 volts then what you had at the battery. You might as well just start right there and test at the starter with the key on start the next time it acts up and if it's ok there, then there is really no need in testing all of them different connections, but if it is low, then at least you know how and what to test. So, if it is ok, then you should move onto testing the negative side, or the negative battery cable. Just place the positive test lead back on the battery and this time move the negative test lead one connection at a time toward ground and the same results as the positive side apply.
When and if you find the spot where the voltage really drops, then from that spot back to the last spot you just tested is bad and if it's a connection clean and re-tighten it. If it's a wire or cable, replace it. If the starter solenoid doesn't even make a sound, then of course you will have to test that circuit as well.