Plug some numbers into Brew 365's or Brewer's Friend's mash calculators. As long as your mash tun can hold the volume of grains and water and is still well stirable a W/G ratio of 1.25-1.75 will be fine. I've gone from 1.25 through 1.33 to 1.5 lately, easier to stir and to hold the heat a little better. I like to (batch) sparge twice with equal volumes.
Preheat the tun, then once the water is at "strike temp" pour in your grains and stir like you mean it for 3-5 minutes. Take the temp after you've stirred, and adjust if needed. When good, cover with aluminum foil to keep more heat down in the mash, and put the lid on. Let it be for the entire mash time.
Now "strike temp" to me means what the calculator says PLUS 4 degrees to compensate for heat loss during stirring while the lid is off. I go to plus 6-7 degrees for smaller (3 gallon) batches, they lose relatively more heat.
You need to know roughly what your boil off is per hour, and estimate what kind of "losses" you get from trub left behind and racking losses, so you end up with 5 gallons to package. For a 5 gallon batch I always brew 5.5 gallons to compensate for my losses and end up with 5 gallons in the keg. If your boil off is 1 gallon per hour (very nominal) you'll need 6.5 gallons of wort, pre-boil for a 60' boil. 7 gallons for a 90' boil.