I use a magnetic mixer to mix all solutions.
Ipurchase ferric oxalate form B&S and mix it in distilled water for sevral minutes. I then turn of the ixer and let is settle for a minute or two , then turn it back on. By the time all chemicals are added most are mixed. The rest usually dissolves sitting quietly over night in the bottle.
By the way, some one mentioned they used boiled water. Boiling water concentrates any material which happens to be in it because the water boils away first.