I am by no means an expert on the subject, but I am sure it works just like when you sign a player, in terms of negotiating with the club and the player. It is up to the club to decide how beneficial the loan will be for them and therefore what percentage of the player's wages they are willing to pay while he plays elsewhere. It is then up to the player to decide whether he wants to play at that level and accept the offer. So I imagine it works something like this:
1-Alty put in an offer to the club to loan a player, stating what percentage of the wages they want to pay and the duration of the loan.
2-If the offer is accepted, it is then up to the player whether he wants to leave his current club and play at that level.
3-If the player accepts the offer, then his two clubs, pay a percentage each towards his wages (unless his new club is paying 100%)
For the player and the club to accept the loan offer, I suppose it depends on the players current wages and situation. If he's not playing first team football and is on a relatively low wage then both clubs can benefit from his new club paying 100%
Hope this is right and helps out