Hi kounavi,
Bjarte's approach is very much suited for people who have already a year or a few years of experience.
But you have apparently just started and thus have different challenges and needs.
If you're close to a competition, there is not much of reprogramming you can do, so you have to style some warm-up for the time being.
I agree with Bjarte that doing long apneas' as a warm up is NOT advisable because you use up those deep buffers you want to have for the max.
So what do you need?
1) I think you first need relaxation and focus.
2) Second you need warmed up muscles.
3) Getting prepaired for some high CO2 and contractions
In your preparation you can combine 1 and 2 and do that in the water by doing some slow freestroke swimming on the surface (warm up), than some short apnea's 25m + turn, where you focus on technique and relaxation.
Now for 3 there are more options. One is doing some dry breathholds on land or in water (CO2 schedule (Tabel A - if I recall right) ) or you can do the 25m laps and do something like 4 x 25m with just 3 breaths in between. This will give your body enough O2, but the CO2 will rise resulting in some contractions and you getting used to high CO2.
When you're ready with your warm up,
get out, drink water, warm up, have everything ready, set a timer and try to take a quick nap until -10 min before your 'Official Top'. Get up SLOWLY and do everything slowly as you move and go to your start. Breath slow, and not deep. Try not to hyperventilate too much before your final breath and know that CO2 is your friend that helps to save O2

Swim relaxed, and just feel the water flowing. Stay with your mind in the 'now', don't judge or guess how far you'll get. If things feel 'bad' you can say to yourself "ok, it can be better, let's see where we stil can get." and return to feeling the water.
If you can test your warmup before you do the competition, take note how long it is so you can plan your competition better. - I know it looks like compeditors are doing nothing arround the pool, but everybody got his own preparation and timing.
The advantage of announcing low figures is that you have more time to observe the others
Have fun, and I love to hear back what works for you!