Step 1: Materials
Vise Nuts and Bolts
A Rubber Mallet (Or piece of wood. Or your hand. Just something softer than the vise)
Dial Test Indicator
And a collet (that fits the dial test indicator)
If, like me, you're not lucky to own all this equipment, you can get all of this and more with a TechShop membership!(http://techshop.ws) (I made it at the TechShop)
Step 2: Clean Everything
Take a shop towel and wipe down the table bed. Errant chips will throw off flatness and may mar the table or vise surface. If the vise is going to be in position for a long time, coat the table with a little oil to prevent rusting. Clean the bottom of the vise with a shop towel and place vise on table. Put t-slot nuts and bolts in position, but leave them loose.
Step 3: Roughly Square the Vise
With both hands, slide the vise toward and away from you to get a sense of it's range of motion. Try to center the vise within its range of motion. If one side hits maximum during adjustment, you'll have reset and start over. To save time during adjustment, use symmetric features on the vise to roughly square the vise with the t-slots.
Snug the t-slot bolt on the left side of the vise. A good feel for snug only comes with practice, but try turning the bolt until you feel the table and vise make contact, then maybe a quarter turn more. We want it snug enough that the vise doesn't slide, but not so tight the vise can't rotate slightly when we tap it with the hammer. The closer your initial alignment, the tighter you can secure this bolt initially. Tighten the right bolt so the table and vise just make contact, plus maybe a 1/16 turn. We want the left side mostly fixed, with the right side just loose enough that it'll allow the vise to rotate with a firm tap.
Step 5: Load the Dial Test Indicator
Place the Dial Test Indicator in the appropriate collet. Depending on your dial test indicator, it may have a different size adapters.
Note the default position of the needle on the dial. Most indicators have about .030" of travel and we want to be in the middle of that range, so rotate the dial so the zero is half the total travel (.015" in this case) away from the resting position of the needle.
Adjust the X, Y, & Z axises of the mill to place the ball end of the dial test indicator at the top left interior of the back side of the vise.
Feed the table towards you until the dial test indicator reads zero.
Feed the X Axis until the dial test indicator is on the right side of the mill. Watch the needle as the indicator moves along the vise face. If the needle travels more than .010" before you reach the right side of the vise, stop and continue to step 9.
Step 9: Square the Vise
Tap the front corner of the vise until the dial test indicator reads zero. The taps should be like a really hard thwack of your fingers against a table. If you have to hit the vise really hard to make the needle move, the t-slot bolts are too tight.
Step 10: Check Squareness and Tighten Vise
Feed the dial test indicator back to the left side of the vise. The dial should stay at or around zero for the whole duration. Pits and burrs on the face of the mill will show up as low and high spots, but you shouldn't see any tendency to increase or decrease. If the vise is within squareness tolerances (the above shows .0005), tighten the vise to operating conditions and remove test indicator. If you're out of tolerance or didn't make it all the way to the right side of the vise in step 7, tighten slightly, re-zero left side and repeat steps 8-10.
Congratulations, you've got a square vise!
You can try this out at the TechShop (http://techshop.ws) or check out other instructables made at the TechShop at https://www.instructables.com/group/techshop or search for the phrase "I made it at the TechShop".