Some insight on the combined feeding method from my experience

Exclusive breastfeeding is the best your babies can get, primarily during the first 6 months of their life, but some mothers experience low milk supply due to various causes and won't be able to cater to their infant's nutritious demand. In such circumstances, formula milk feeding is the perfect solution.
Premature babies especially won't have the strength to latch on or suck the breastmilk at first. They require time. Until then they should be fed formula milk to maintain and increase their weight.
In my situation when the baby was in NICU, the doctor had prescribed the "Nestle pre nan" formula milk and am continuing the same brand. So the type of formula milk which suits your baby would be recommended by your doctor.
Formula milk feeding pattern:

Preemies up to two or three months of age are fed milk every two hours. In NICU they maintain a pattern like this but what about it after returning home?
Remember to always breastfeed first! Even if the baby is not sucking or not properly latched, try your level best. It will continue for a few days and they will eventually and naturally learn.
If direct feeding fails, express your milk. Express as much as you can and feed it in a paladai or normal spoon.
Breastmilk digests fast and they are soon hungry. After about half an hour feed the formula milk max about 30ml initially.
Once fed keep a gap of about 2 hours and start with the next feeding session same as above.
Continue the same dosage until your baby is nearly 2 months of age.
How about the dosage after 3 months of age and what if they start taking breast milk normally?
If your baby has started to nurse well it will reflect on their weight gain. Loss of weight, hard stools, constipation and continuous crying are signs that they are not getting enough milk. Some mothers see a sudden increase in milk supply and some not! In my case, milk supply had a medium flow entirely and formula milk feeding was a must.
Even if you are low on milk supply, never mind, demand feed your baby if breastfeeding. By now you would surely get an idea of your baby's appetite. Three months of age and after that, you can start increasing the formula milk dosage.
If your baby is continuously nursing for an hour or more and dozes off. If shows they are full and content.
If not, try feeding the formula milk in increased doses. Start from 60ml. Can go up to 120ml per day depending on their satiety levels.
Sometimes they may even ask for more or sometimes less. There is nothing like a fixed dosage of formula milk to be fed.
Until the age of 6 months, babies should be fed both breast milk and formula milk simultaneously. After that, you can cut down on formula milk feeding a bit as they are on solids but breastfeeding should be continued normally.
Few things about formula milk feeding:

The amount of water you use to mix the powder should be precise. Example: 30ml of water for 1 levelled scoop and 2 scoops for 60ml. Use the same scoop provided in the packet and keep a measuring jar for water always handy.
Why it's important to maintain the extract dosing of water and the powder?
If the milk prepared is very dilute it won't provide adequate nutrition to the baby and concentrated milk can make the baby gassy and constipated.
Maintain proper hygiene of the utensils being used. Use hot water to sterilize it after every feed. For feeding prefer steel or silver spoon or paladar. Glass or steel bottles are best if using a feeding bottle.
When You're preparing the formula milk, water should not be too hot nor too cold. Normal hot water in which the powder mixes lump-free is good.
After preparation, don't shake the formulation. Shaking leads to bubble formation and may cause bloating or gas formation in the babies.
You can prepare hot water and store it in a clean flask for the entire day's usage.
Once You have prepared the milk don't keep it long for more than an hour. If the child refuses, discard and prepare a fresh one.
Stick to one particular brand of formula milk. Your babies will have adjusted to it and changing the brand may lead to changes overall.
Formula milk is fed to babies who are not being exclusively breastfed due to various reasons. It's not to discourage breastfeeding. Mother's milk is always the best for babies. So even if you are low in supply feed the baby or Extract it. Extraction maintains a steady flow in the milk supply. Formula milk is nearest to breastmilk but devoid of antibodies so even if the quantity is less, the first preference should always be breastmilk.
Formula milk should be an alternate option when there is no other choice but not otherwise as a deliberate option.
My son is 21-month-old now and already on baby-led weaning! He just doesn't like to waste his precious time breastfeeding in the day! I nurse him as much as possible in the nighttime. Would continue the same until my milk supply stops naturally.