This is how it is done. 2 step process. To start, get a bunch of fodder with "junk affixes". All you have to do is use junk low level affixes stuff like "might I". Can also use augmentation aids if needed, goal is to just add slots, type of affix does not matter. Then stop when you have to right amount of slots. For step 2, Then you put on the affixes you actually want for that weapon.
If you have low augments like "might I" then they transfer 100%, except there is a penalty when upslotting so it would go down to 90%. You can safely get around the penalty (remain at 100% success) by using the support item "augmentation aid +10/20/30/40%". With this strategy you can upslot to 5 slots, all 100%, assuming you have both low level augments, and assuming you have a 40% aug aid when upslotting 4 slot to 5 slot, (If you didn't use the 40% booster for 4 to 5 slots it would be 60% success). If you went from 5 to 6 slots using a 40% aug aid and same affixes they would all be 95%. So you can gamble on 6, but with 5 slots you can guarantee it will be a success.
Also In the future they will add more higher % aids and also have affix boost days for added 5-15% success.
I personally stick to 5 slot for now to stay safe, as the final can always be 100%, and 5 slot fodder is more affordable then 6 slot.
Also, Try to have 100% success for the Final affix if your gear, if you want to stay safe-only because If it fails you will loose those fodder and affix booster so it will be a big waste of resources and meseta. Personal choice. Many would gamble on less than 100%, which I think is fine for hurried upslotting, but not for the final upslot.