Hi there, First time mum expecting to have baby this December. I have pure cotton onesies that are without sleeves or pants. Do I dress baby in one of those and then wrap he/she in a muslin baby wrap? I'm so confused with how to dress baby for bed and what blankets to use for a newborn this summer.

it can be difficult to work out in advance what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rath than trying to modify the room temperature with heating or cooling.

I wish I could give you some specific guidance for dressing a newborn, but every baby is different.
Red Nose has the following information.

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold. Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.  - Always sleep baby with head & face uncovered.

Can we say how many blankets or bedding to use when placing baby for sleep? – No.

Red Nose recommend that rather than state how many bedclothes can be safely placed on a baby, parents can work out the amount of bedding to be used after considering these factors:
The room temperature where baby is sleeping.
How hot does the baby feel?

A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Whether the baby has a cold or infection or another special need.
Consider how many layers that you as the baby’s carer are wearing comfortably.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

A useful guide:
Dress baby for sleep using layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold - add/remove lightweight blankets to ensure baby’s chest feels comfortably warm to the touch

A good way to check baby’s temperature is to feel baby’s chest or back of neck, which should feel warm (don’t worry if baby’s hands and feet feel cool, this is normal).
Ensure baby’s head is uncovered - no hats, bonnets, beanies or hooded clothing.
https://rednose.org.au/article/how-much-bedding-does-baby-need

As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.