how many sat/byte do you use for transactions?
Use the websites suggested by Charles-Tim above the check the mempool and get a estimate of the low, medium and high priority feerate.
Do you increase and decrease it depending on how quickly you want the transaction to be processed or how big or small the transactions value is?
Yes, your feerate determines how fast your transaction gets confirmed. The transaction value does not influence the fee paid to get it confirmed.
Do you use a fixed amount for transactions?
No, that's not advisable.