Honestly, it's just practice really.

Hit up some YouTube videos, watching someone else do it will probably help more than hearing it described.

Straighten the wires by pulling them against a pencil or pen.

Line them up in the correct order and hold them close between thumb and forefinger.

Cut the ends to be the same length.

Then attach the connector carefully. Pushing the cables against the top or bottom a bit as you insert them should help keep them straight.

It's a skill you develop with time.... I guess, cause I haven't developed it yet.

Things that usually help me with this: Straighten the wires while putting them in order

If cables are too stiff, you can always either cut their tips or use a plier

Pressing with a finger can help you hold the wires in order

While ordering them, you can try the wires on the rj45. It has little guides that help you straighten the wires.

Lining up the wires, ensuring they’re straight and making sure they’re trimmed to the same length will help avoid crossover too.

You can help straighten them on the square edge of a table, just press them between your finger and the table at the part that’s stripped from the insulation, then pull them over the edge applying pressure the whole time.

You can also look for the newer cat 6 connectors. Lots of brands have an insert that you can slot the wires in to before putting them in the housing, which helps a lot.

Example here: https://www.amazon.com/W-NECTOUN-100-PACK-Connectors-Ethernet-Connector/dp/B0B1DHQCP7/

If it's a premade patch cable....good fuckin luck. It's possible but they're usually too thin and flexible to recrimp without a bunch of attempts. Bulk wire is a lot stiffer and maintains its shape.

Are you using pass through (EZ) connectors? That makes it a ton easier.

Like others said it's mostly just practice.

What helps is to align the (short) ends and hold them flat between your index finger and thumb. Use your free hand to get them in order. Once they're in order, keep holding them still between your index finger and thumb using one hand, then use your free hand to slot on the connector

Edit: also bending them back and forth a bit will soften them up and reduce them curling in all sorts of directions. It also weakens them, so don't overdo it (mostly only works for solid cable, the type meant for permanent installations like inside walls)

My trick is to wiggle them back and forth a few times between your thumb and index finger while pulling on them just a bit. This warms them a bit from your hand and the bending. And the push through connectors are a godsend since you can strip them a little long.

Make sure you're straighting them out before hand with the tip of a screwdriver makes it easier. Also look for rj45 connectors with a load bar, makes it a lot easier since you can feed them one at a time and make sure they are aligned properly, often they also come with pass through connectors and you should be able to do that in no time.

There are more expensive but easier RJ45 head (like 3 pieces design). Some design requires tailored made crimper and such crimpers are more expensive.

Edit: for example https://www.amazon.com/3-Piece-Shielded-Connector-Modular-Network/dp/B071GXNGD3