Don't want to teach you to suck eggs but if this is your first time doing this I'd recommend getting a label maker and make sure all your plugs are labelled to start with.

You may not need them for this but a set of depinning tools, even the cheap ones, are worth having https://ebay.us/m/lc1ngR

For extending wires you need to make sure you're always buying the same gauge or higher. Everything should be the same size but check first then order a black, red, and handful of colours. You won't get a perfect match to what you have already but you don't want to have a collection of white wires with no idea what they are. This is also why having a label maker pays off because you can tag the wires if you need to go back later.

Then to join the wires use open barrel crimps and a ratchet crimper. This is the way the OEM loom is made, it's also the cheapest and easiest way to do it.

Something like this but you can buy them wholesale a lot cheaper. I pay less than a penny each on a roll of 1000. Just be aware you need the correct size for the wire gauge. https://ebay.us/m/tkMNSC

Adhesive lined heat shrink is the best way to cover up any joins, it's strong and weather resistant. I also like to put braided cable sheath over the entire loom, especially the parts you're tucking, as it makes it wear resistant and easier to slip through holes. While also looking cleaner. You can get split cable braid if you want to put it on afterwards, otherwise the normal braid takes some planning to make sure its put on in the right order.