Careful - this can lead to be an obsession!
The 'scope is easy because it isn't a precision instrument - you're looking at 2% or so horizontal and vertical for a 2235 to get it within the Tektronix specification.
For the timebase get a 10MHz TCXO from ebay and couple it to a couple of ÷2 and ÷5 counters (74xx90 or equiv) to get you 10,5,2,1MHz -etc the David Bridgen link takes this a bit further and adds a pulse vs 50% duty output as well.
A decent TCXO should be accurate to 1ppm to 0.1ppm so that's way more precision than you need for the timebase and should do for checking the sig gen master oscillator and the frequency counter.
If you want better than that then a rubidium unit or GPS disciplined oscillator is the way to go for a frequency standard - that will get you into the 0.1ppb (or a little better) territory. Unfortunately prices have gone up a lot recently.
For voltage standards get a
DMMCheck - or one of Doug's other reference sources, that will give you some idea of how the 8050A is doing (but might not be enough to adjust it) on at least one voltage and current range and three resistance ranges.
The 2235 has one principle gain pot per channel so you can check the vertical deflection with a DC source such as the DMMCheck.
Or look at the "DMMCheck plus" which has some square wave outputs.
The two pots are towards the front left of the 'scope under the CRT. Don't fiddle with the front end attenuator board pots unless you are feeding in known clean square waves and it looks like the transient response is off (and then have a read of the manual - you need a square wave source with fast edges that can do a couple of frequencies - 1kHz, 10kHz, 50kHz and 1MHz from memory).
You can now check the amplitude accuracy of the sig gen - up to a point.
You don't say what sig gen you have or whether it's a "function generator" or "signal generator". Assuming "function generator" means low frequency (typically max of 50MHz or lower) sine+square+triangle outputs with relatively limited amplitude range then the spec is likely to be 1-5% on amplitude. Also, assuming "signal generator" means a beast intended for RF testing with higher frequency (> 100MHz often >1Ghz eg Marconi 2019, 2022, 2024 etc) but sine only output then the accuracy is more likely to be 1dB (12%) or so.
So, if you check your generator output and it looks to be within 5 or 10% over a range of frequencies chances are your 'scope and gen are close enough. If it's off you won't know whether it's the 'scope or the gen but chances are you'll be OK.
I have no affiliation with voltagestandard.com except that I bought a DMMCheck from them and was perfectly satisfied - my only regret is that living in the UK I can't really take advantage of the free re-checks that they offer.