If you’re concerned about cost, the cheapest way to add insulation under your existing sleep system is a closed-cell foam (CCF) pad. Fancy ones from Therm-a-rest can be $20 (Ridgerest) to $35 (Z-Lite) but, here in the USA, Walmart and budget stores have CCF pads for $8.
Even before that, bring a really warm hat you can sleep in.
In the skills-not-gear area: Don’t set up camp in a low spot. Cool air pools in low areas, especially on cloudless, windless nights. Just being 5-10m higher on a low rise can make it 5C-8C warmer. Look around and think, “If the lake/stream/pond rose 10 meters, where would I want to be?” and camp there.
Also: if you can camp under trees, you and your tent will lose less radiant heat to deep space (actually, it’s that the tree radiants that infrared back at you). That makes you a bit warmer and greatly reduces the condensation on your tent and gear.
Keep warm clothes – at least an extra layer for your top, bottom and feet – close by to put on at night if you feel cold or if the temperatures drop.
Make notes on the temperatures, what you used, and how well it worked. Over time, you’ll develop a good sense of what you require to sleep comfortably.
Realize that in winter, you have less daylight and more night. Do you have activities (eBook, music, etc) to pass the time inside your sleeping bag? Just sitting still watching the scenery or the stars may not keep you warm enough once you stop moving.
In in winter, consider the batteries in all your gear (water purification, headlamp, etc). Lithium batteries work down to -40C but alkaline batteries fail not much below freezing.
If you cook with butane canisters, look at any of the many threads on BPL about how to keep them going at -10C, -20C and even -30C (I’ve done that).
Spending a night outside on your porch at home is always a good idea as you learn to winter camp – if you’re not comfortable there, you won’t be comfortable in the higher elevations in the wilderness.