I think the advice would depend on if you plan to keep breastfeeding/combi feeding long-term or if you are planning to switch to formula full-time, but want to do it gradually. If you don't plan to stop BF entirely, I'd have a look at kellymom or la leche league to see what advice they give you on keeping your supply up and how to do it.
I can say from my own experience though that we started to combi feed at 6 weeks but totally from a bottle, with expressed milk and formula as needed. It's slightly different because I started to combi feed because of existing health issues of my own, so my supply was probably not great to begin with. But we took the route of breast milk first (if I had any, I started to express then, so was trying to get as much as I could but it wasn't always a full feed) and then topping up with formula if she was still hungry. I think honestly it's probably easier just to either BF or FF rather than to top up because then you only have to do one and not both. My daughter had no issues with formula and happily took it so we didn't need to do it gradually, but I was just trying to still get as much BM as I could in her.
But if you want to maintain your supply, you will also have to express at every feed that you offer formula. If you want to wait until later on, closer to 3-4 months, your supply will be more established and you probably won't have to, except for comfort. But if you don't want to stop BF altogether at this point, you'll need to express as often as she would feed anyway (either expressing while you give her a bottle or after if she falls asleep and you can put her down or get the pump on while holding her). This bit is really key. I personally was pumping 6 hours a day (30 minutes per side, 6 times a day), which sucks, and I would recommend a double pump if you want to go that route (which would make it about 20 minutes total per pump, as often as you give her a bottle and she doesn't have a feed from you). Even doing this, I lost all my supply within a month and had to switch totally to formula at 10 weeks.
So my advice would probably be, if you can manage to wait a bit longer, to wait until your supply is established, closer to 3 months before you start to combi feed if you plan to continue BF along with formula. It will save you a lot of extra effort with pumping, which is a lot of work, though not impossible. If you don't want to wait, just make sure you're pumping regularly whenever you don't feed her from the breast, both sides, at least 20 minutes. If you do plan to switch totally to formula and you aren't concerned about lowering your supply, I would just switch out a BF with a bottle of formula, starting slowly so you are comfortable, maybe once a day to begin with and see how it goes. You might end up keeping your supply up for quite awhile without much effort if you have a good supply anyway, or you might be like me and have it decrease pretty quickly. It really just depends what you're long term plans are.